Tank Gun Systems: The First Thirty Years, 1916–1945: A Technical Examination

By William Andrews

Much has been written about the use of tanks in battle. Little, however, has appeared about the gunnery systems that are at their core. This book describes and examines the main gun systems of medium and heavy tanks from first use in 1916 in World War I to those fielded in numbers to the end of World War II in 1945, including tanks of the interwar period. Specifically considered are guns of a caliber greater than 35 mm, which have been deployed in numbers greater than 100. The emphasis is on guns mounted in turrets on heavier tracked armored fighting vehicles (greater than 15 tonnes) which were considered tanks. There are, though, exceptions, in that the naval 6 pounder guns in First World War British tanks, as well as the 75 mm guns in French medium tanks of the same period (all turretless) are included.

The treatment of gun systems includes sighting and fire control equipment, gun laying equipment, mounts and the array of munitions fired, as well as the actual gun, including its, barrel, cradle, breech, firing mechanism, sights and recoil system. Related to this are issues of gun handling (loading and unloading), ammunition design and rates of fire. Also examined are the maximum impulse and energy generated by firing some of the munitions available that must be absorbed by the gun recoil system.

• Language: English
• Publisher: Pen and Sword
• Release date: Jun 30, 2023
• ISBN: 9781399042376

William Andrews

William Andrews received a Bachelor of Chemical Engineering degree from the Royal Military College of Canada (RMC). He subsequently completed a master’s degree and doctorate in nuclear engineering, also at RMC. After graduation, he served in the Canadian Army in the armor and the Royal Canadian Electrical and Mechanical Engineering Branches. His regimental duty included tours with the 12ème Régiment Blindé du Canada, the Royal Canadian Dragoons and the 8th Canadian Hussars and at the armor School. After completion of his doctorate, he retired from the army and joined the faculty of RMC, where he taught chemical engineering and military science topics at the undergraduate and graduate levels. The military science courses included ammunition and weapon design and ballistics. He is now fully retired and is a Professor Emeritus of RMC.

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INTRODUCTION

Chapter 1

Introduction

Tanks first saw operational service at the Battle of Flers-Courcelette (part of the Battle of the Somme) on 15 September 1916, with the British Army. Their use was adopted later in the First World War by France, the United States and Germany. Since then, tanks have been produced around the world and continue to form a core component of most armies.

It has long been recognized that the three design aspects of tanks are protection, mobility and firepower. Different vehicles have exhibited different balances among these three. In fact, until recently, protection (vehicle armour) tended to vary inversely with mobility. That said, most nations mounted the largest/most powerful main guns that could be accommodated and serviced in the vehicles available. This problem or challenge became more acute when there was a need or desire to increase the size and power of a gun system on a platform or chassis that had not been initially designed for it.

Much has been written about the use of tanks in battle. Little, however, has appeared about the gunnery systems that are at their core. This book will describe and examine the main gun systems of main battle tanks of the first half of the twentieth century. Descriptions of mobility and protection, as well as operational and tactical employment, can be found elsewhere.

In order to provide some realistic limits to the discussion of tank armament, concentration is on the main armament, or main gun, of tanks. Consequently, secondary armaments, such as machine guns, smoke grenade dischargers, mortars etc., will not be addressed. Another somewhat arbitrary restriction is that only guns of a calibre greater than 35 mm, which have been deployed in numbers greater than a hundred, are examined. These limits have been imposed in order to be able to deal with the variety of guns fielded over the timespan considered. A consequence of this decision is that some topics are excluded, including experimental guns, or guns produced in small numbers, guns mounted in casemates (e.g., assault guns, or primarily artillery guns on tank chassis), the armament of light reconnaissance vehicles (with or without turrets), of armoured cars (wheeled vehicles), and of turreted tank destroyers (e.g., the M10, M36 or M18 of the US Army). Rather, the emphasis is on guns mounted in turrets on heavier tracked armoured fighting vehicles (greater than 15 tonnes) which were considered tanks. There are, though, exceptions, in that the naval 6-pounder guns in First World War British tanks, as well as the 75 mm guns in French medium tanks of the same period (all turretless) are included, in spite of the fact that, in the case of the latter, they were artillery guns mounted in vehicle bodies, e.g., the 75 mm guns in the St Chamond tank.

At the outset, it is worth noting that information on a number of systems is sparse, at best, so some descriptions will be in much more limited detail and depth than others. Also, the term ‘gun system’ can be quite broad, and here it had been decided to include sighting and fire control equipment, gun laying equipment, mounts and the array of munitions fired, as well as the actual gun, including its barrel, cradle, breech, firing mechanism and recoil system. Related to this are issues of gun handling (loading and unloading), ammunition design, rate of fire, and consistency and accuracy. Not all of these will be addressed for each gun discussed. Further, the manufacture and metallurgical compositions of guns are not included.

This book does, though, examine the basic components of the gun systems under discussion, as well as determining, where possible, the maximum impulse and energy generated by firing some of the munitions available that must be absorbed by the gun recoil system.

Terminology for gun systems varies from country to country, with notable differences between British and American usage. This book will attempt to explain the choice of terms used and, where possible, to point out discrepancies between the two approaches.

Finally, the book is divided into chronological sections, i.e., the First World War (1915–1918), the Interwar period (1919–1939) and the Second World War (1939–1945). Although a number of gun systems developed and fielded during the Second World War continued in service well into the post-war period in newer tanks, they are only described as they appeared during the war. An example of this is the British 76.2 mm Ordnance QF 17-pr L/55 gun, which is described as mounted in the Sherman IC and VC tanks, but not as it subsequently appeared in the Centurion Mk 1 tank. Another example is the Soviet 122 mm D-25T L/43 gun mounted in the JS-2 tanks, but not as it subsequently appeared in the post-war T-10 heavy tank. Further, the Soviet 100 mm D-10 L/53.5 gun mounted in the SU-100 tank destroyer during the war (but later deployed in the postwar T-54 and T-55 tanks) and the American 90 mm M3 L/53 gun mounted in the M36 tank destroyer during the war and the M26 tank late in the war (and subsequently in postwar M46, M47 and M48 medium tanks), are not addressed.

Chapter 2

Components of Gun Systems

General

At its most basic, a gun, of almost any type, is composed of a reaction chamber, where stable propellant is induced to undergo a chemical reaction to generate propellant gases, which are used to propel the projectile down the gun tube towards a target.

Consequently, then, the two main components are the gun barrel and the chamber, which are illustrated in Figure 2-1. The barrel has an open end at the muzzle and is attached at the chamber end to a breech. This latter is opened to allow loading of both the projectile and the propellant, and closed during firing. Different breech arrangements or designs will be discussed in greater detail below. These characteristics are common to guns of all natures.

The barrel is often referred to as the cannon, or ordnance, and the breech contains a fixed portion, called the breech ring, which attaches the breech to the cannon, and a breech block, which allows the breech to be opened and closed for loading and firing, respectively. Further, the breech ring and block contain the firing mechanism, which ends with a firing pin or striker making contact with the primer of a round in the chamber. Also of interest with respect to the barrel is the rifling, used to impart spin to a projectile as it travels down the barrel. Barrels came also to have muzzle brakes attached to mitigate the effects of recoil. Finally, there was often an internal lock to immobilize the gun during administrative movement, in order to prevent wear on the elevating gears.

The gun itself must be placed on or in a mount. For artillery guns, these mounts are generally light, to allow mobility and ease of handling, and are referred to as carriages. When mounted in tanks, this requirement is not nearly as limiting. In fact, the mounts are fixed to the vehicle and thus stresses generated during firing are transmitted to and absorbed by the host vehicle. This applies, of course, to both tanks and self-propelled artillery guns, as well as to naval ships.

The basic components of a tank gun are shown in Figure 2-2. The gun barrel/ breech combination is mounted within a cradle, within which the barrel moves. The cradle is attached to the vehicle structure at the trunnions, about which it (and the gun and breech) pivot vertically.

The recoil and recuperation systems are attached to the gun/breech and the cradle.

Figure 2-1 Principal components of a gun barrel. This simple diagram shows a monobloc barrel, but does not show the distinct breech ring.

The gun is moved horizontally, or in azimuth, by rotating the entire turret, with the exception of guns mounted in sponsons which were then attached to the hull, but allowed only limited gun traverse.

Figure 2-2 Photo of the 17-pounder gun system installed in an Achilles open-topped self-propelled gun. Although used by artillery units in the Second World War, the gun is the same as was mounted in Sherman tank models IC and VC (or Ic and Vc). This photo is used as an illustration of the gun mounting, since the mounting within the Sherman turret is not as visually accessible. This is the Achilles on display at the Tank Museum, Bovington, UK. (Author’s photo)

Interestingly, the same gun, mounted in the Archer, another open-topped British-designed self-propelled gun, has the recoil cylinders mounted above and below the gun (undoubtedly due to the more confined hull of the Valentine host vehicle), while in the Sherman they were on opposite sides of the gun but off the horizontal.

Although it is not the intention of this book to discuss artillery systems in any detail, except where they have common aspects with tank guns, it is noteworthy that artillery guns have been mounted directly into tank chassis, such as the French St Chamond, which had two versions of the 75 mm field gun mounted directly onto the vehicle’s interior floor.

Gun Mounting

Most tank guns were mounted within revolving turrets. The gun was mounted within a concentric cradle, which in turn was mounted to a mantlet. The cradle and mantlet were mounted to the turret at the trunnions, which allowed the gun, cradle and mantlet to be moved/rotated in the vertical plane. The gun was moved horizontally by traversing the turret about the hull, usually using a turret ring composed of teeth and bearings. The turret ring itself was constrained or limited by the width of the tank hull. The consequence of this was that the size of the turret ring often limited post-design expansion to accommodate larger guns that required larger turrets. Examples of these aspects of gun mountings are shown above in Figure 2-3.

Figure 2-3 Photo of a 75 mm Sherman V of the 22 Canadian Armoured Regiment (Canadian Grenadier Guards) that received a direct hit on 10 August 1944 at Hill 195 near Falaise. Of note is the turret, which was dislodged and landed upside down on the hull. In the photo, the turret ring and mantlet can be seen. (RCAC Museum)

At this point, one can now look at the various gun components in more detail.

Cannon

The cannon, or gun tube, had a number of functions. It provided the chamber for propellant reaction, and then guided the projectile along its length. This both allowed the pressure generated by the propelling gases to accelerate the projectile along the barrel and was also used to impart a spin to it. This spin was essential for stable flight from the gun muzzle to the target and was imparted by the interaction of the round, and more particularly its driving band, with the rifling within the barrel. This latter is a series of spiral ridges, called lands, that engaged or cut into the soft driving band material (often copper) to make the round spin. The rifling can be seen in Figure 2-4 and the driving band is shown in Figure 2-5.

As mentioned, the cannon allowed the high pressure gases to accelerate the round down the barrel. The pressure was highest at the chamber, where the propellant combusted. As the round travelled down the barrel, the space for the propelling gases increased, and so the pressure fell. As a result, gun barrels were thickest at the breech end and became progressively thinner toward the muzzle, as can be seen in Figure 2-1.

Figure 2-4 A view down a barrel, showing the rifling (the lands or raised ridges, and grooves or recessed regions). (min.news/en/military)

This has been addressed a number of ways over time. Initially, barrels were uniform in thickness and designed to withstand the highest pressures generated. With time, barrels were made thinner, but were strengthened at the chamber by either wire wrapping (especially for naval guns – now obsolete) or by using layers (jacketed). Figures 2-6 and 2-7 provide examples of jacketed barrels.

Figure 2-5 Images of 76.2 mm 17-pounder projectiles in cross section, showing the driving bands. The projectiles shown are, left to right, armour-piercing, armour-piercing capped and armour-piercing capped ballistic capped. (theshermantank.com)

Figure 2-6 Drawing and photo of jacketed gun barrel (and the prominent muzzle brake). The 88 mm KwK36 L56 jacketed barrel is mounted on the Panzerkampfwagen VI Ausf E (Tiger I) at the Musée des Blindés, Saumur, France. (Author’s photo)

Note: either the muzzle brake or the barrel (or both) is/are mounted incorrectly, as the openings on the side of the brake should be horizontally aligned (as in Figure 2-7).

Barrels have also had attachments added to them. Most prominent is the muzzle brake, examples of which are shown in Figures 2-6, 2-7 and 2-8. These have been added to reduce the recoil forces exerted on the trunnions and, by extension, on the recoil system and the vehicle structure. They also served to direct propellant gases sideways to prevent them contacting the ground to stir up dust and obscure the crew’s view of tracers at the base of projectiles in flight.

Figure 2-7 Muzzle brake on end of KwK43 L/71 88 mm barrel on the Panzerkampfwagen VI Ausf B (Tiger II) at the Musée des Blindés, Saumur, France. (Author’s photo)

Figure 2-8 The action of propellant gases impinging on the baffles of the muzzle brake after the projectile leaves the muzzle are shown.

Figure 2-9 Barrel end cap on 76 mm M1 gun on a M4A1 tank at the Musée des Blindés, Saumur, France. (Author’s photo)

Figure 2-10 Barrel counterweight fitted to the muzzle of the 75 mm M2 gun on an American M3 tank. (albumwar2.com)

Yet another addition to barrels was an annular cap, fitted (often screwed) at the muzzle. This was used to strengthen the barrel at the muzzle, where the round exited the constraints of the barrel and entered the disturbed area of gas muzzle flow, which could result in higher wear at the muzzle. Another application of this cap was as a counterweight to help balance the barrel about the trunnions. With much of the weight of the barrel at the chamber and breech end, shorter barrels would be more difficult to elevate or depress if they were greatly out of balance. A third use was to protect the threading at the muzzle until brakes became available, as was the case with early examples of the American 76 mm M1 gun. Examples of barrel caps and counterweights are shown in Figures 2-9 and 2-10.

Breech Systems

There are two basic types of breech used on large calibre guns – screw breeches and sliding breeches. Before describing these, however, it is useful to note that the breech must not only be used to allow loading individual rounds of ammunition, including both the projectile and the propellant, but must also provide obturation, or a seal to contain the hot high pressure propellant gases within the chamber after ignition.

Most of the tanks described below used fixed rounds of ammunition, which had the projectile crimped or fixed to the shell casing (often brass). On propellant ignition, the casing expanded and provided the gas seal for the breech. This, in turn, allowed the breech block to slide and provided for a higher rate of fire. That said, the casing had to be ejected from the chamber and then disposed of (usually thrown from the tank at the first available opportunity). Until recently, sliding breeches, firing fixed ammunition, have predominated in tank main armament systems. An example of this is shown in Figure 2-11.

A hybrid type of rotating (sometimes referred to as Nordenfeld) breech was used in the French St Chamond tank of 1917, on the soixante-quinze, or 75 mm cannon, where the breech was opened and closed by a revolving motion and fired fixed rounds.

Interestingly, screw breeches, featuring interrupted thread sealing, are normally used with bag charges that do not have metal casings. For tanks, requiring a fairly high rate of fire for short periods, the extra handling of the screw breech is problematic. That said, with main armament rounds getting progressively longer and heavier, manual loading of fixed rounds was also becoming increasingly challenging within the confines of turrets.

Another aspect of sliding breeches that will be examined in more detail for individual weapon systems is that they can slide to open either horizontally or vertically. For horizontal opening, the breech block will slide open in the direction opposite from which it is loaded. For example, the British 76.2 mm Ordnance QF 17-Pounder L/55 gun mounted in M4 Sherman tanks had a horizontal breech that opened to the right, with the loader positioned to the left of the gun, looking towards the muzzle. Such an arrangement, however, required more room within the turret, to accommodate the open breech.

Figure 2-11 An example of a turret-mounted sliding vertical downward opening breech on an Ordnance QF 2-pounder gun in a Valentine tank, being loaded with a fixed round (40 mm). (vignette.wikia.nocookie.net)

The other option is for breeches to open vertically. This was more economical in space, but in tanks the breech would have to open downward, as upward opening breeches would limit depression of the main gun, an essential attribute for firing from hull down or defilade positions, where the tank was partially masked from the front by terrain features. Another consequence was that a strong spring was required as part of the breech opening/closing mechanism to elevate the very heavy breech block from the open to closed position.

It should be noted that sliding breeches could be made semi-automatic, in that, on recoil, cams caused the breech to open and extractors ejected spent shell casings rearward. Both these actions help speed up the reloading process, to increase the rate of fire.

Recoil Mechanism

Returning to the gun mounting, in order for the gun to remain stable on firing and be returned to its original firing position for the next round, the forces generated by propellant combustion and projectile launch had to be managed. The gun was allowed to recoil within the cradle. That meant that the gun would travel aft or rearward after the propellant had been ignited. This rearward motion was limited and controlled by recoil mechanisms, called ‘buffers’ by the British. When rearward recoil had been arrested, the gun was returned to battery (its original firing position) by a recuperator, or counter-recoil mechanism, which was often an integral component of the recoil system.

It should be noted that recoil distance or travel had to be limited within the confines of turrets and associated stowage, including ammunition. This distance often had to be limited to less than 25–30 cm, depending on the space available. This limiting of rearward travel was achieved both by friction within the cradle system and primarily by the recoil system.

Recoil buffers have been used since the first tank guns and have not changed appreciably in basic design for a century. The main arresting mechanism was a mechanical spring, with hydraulic (oil) and pneumatic (gas) systems subsequently developed. A typical recoil buffer is shown in Figure 2-12. Basically, one part of the buffer was attached to the gun and the other to the cradle. When the gun recoiled, a piston or plunger moved to stress the spring, exerting a braking force of the rearward motion of the gun. The spring could be placed in either compression or tension, with the reassertion of the spring serving to return the gun to battery. This final motion, or recuperation, could be aided by a purpose-designed recuperator. The final action was to slow (or buffer) the return of the gun to battery, so that the final seating was not violent.

Another purpose of the recoil and recuperator system was to hold the gun in battery when it was elevated, by countering the force of gravity, which tried to draw the gun downward/rearward.

There were two basic approaches to recoil management, or recoil system geometry. One common approach, since the first tank guns, has been to have separate buffer and recuperator cylinders mounted to the gun and cradle. Another approach was to combine the recoil buffer and recuperator functions within a common cylinder, while using one or more of these cylinders as part of the gun mounting.

Figure 2-12 Typical hydraulic recoil cylinder geometry showing the location of components with the gun in battery (top) and having recoiled after firing (bottom). (UK MoD)

Gun Laying Equipment

Gun laying equipment refers to the devices used to position the gun, in both elevation (vertically) and azimuth (horizontally) to be able to engage targets. There were usually two separate components, which could be combined in one controller. Gun elevation was effected by moving the gun up or down about the trunnions. This was traditionally done by using a handwheel that was attached to a worm gear. This mechanical arrangement was gradually replaced by hand controller(s) that were connected to hydraulic drive units or to electrical motors.

Horizontal control is exercised by moving the turret about the hull, with the gun remaining fixed horizontally within the turret. Here again a handwheel was initially used but was gradually replaced by power traverse, again either through hydraulic or electrical power.

Traditionally, again, these controls were manned by the crewman designated as the gunner, who laid and fired the gun. Later, duplicate controls were provided for the tank commander, usually as an override, so that the gunner could be aligned in the general direction of the target, to help with target identification and acquisition, or to actually lay and fire the gun, if necessary.

As tanks became better protected and carried larger guns, they became heavier, with manual controls requiring greater physical effort, or with more sophisticated traversing gearing, more time. This, in turn, led to slower speeds of target engagement.

A drawback of hydraulic systems, though, was that gun movement required hydraulic fluid to be provided under high pressure. In order to flow under these conditions, the hydraulic fluid itself had to have low viscosity (thickness). This, in turn, led to pierced hydraulic lines releasing flammable fluid into the turret and contributing to vehicle fires. By the end of the Second World War, most gun laying equipment used electrical or hydraulic power, often augmented by manual systems.

Sighting Equipment

Tanks use sights, or sighting systems, to determine the alignment of the gun, in both azimuth and elevation, to hit the chosen target. Tanks are primarily direct-fire weapon systems, which means that targets to be engaged are visible to tank gunners. As distances of engagement could vary from very close to up to 2–3 km, magnification was usually used. Initial sighting systems involved telescopes attached to the guns. These gradually gave way to telescopic sights aligned within the mantlet of the tank.

This arrangement, although providing minimal offset of the sight from the gun, also introduced a hole, or weakness, into the mantlet, which was often the most heavily armoured portion of the vehicle. To address this, periscopic sights were introduced that involved periscopes being mounted on the turret roof, to allow the gunner perhaps a better view of the target, since the periscope head would be higher than the gun axis, while providing better protection to the gunner, as there was no longer a weakly-protected or unprotected path along the line of sight of the periscope to the gunner. Further, damaged periscopic heads could more easily be replaced than telescopic sights.

The consequence of the greater security was that the sight now became more offset from the axis of the gun. This was initially addressed by boresighting, where the gun and sight were aligned at a selected battle range. If this were fixed, aiming marks within the sight could be adjusted to account for this parallax.

This sight offset meant that the sight and gun were only truly aligned at the chosen boresighting or alignment point, with parallax introduced for other ranges. Long engagements were also challenged by dispersion (round-to-round variations in the flight paths of rounds) within the ammunition chosen for the engagement, and the diminishing effectiveness of rounds against hard targets at increasing ranges. Both these aspects will be addressed in greater detail in subsequent chapters.

As ammunition beyond fairly short battle ranges had to have elevation and perhaps even azimuth applied to the line of sight to increase the chances of a hit, many sights featured a series of aiming marks that were dependent on the estimated range to the target. These could be simple offset aiming marks, or aided in range estimation by horizontal or vertical pairs of lines that would correspond to a typical target at a certain range.

It is noteworthy that the sights described above are optical in nature, involving combinations of prisms and lenses that operate in the visible portion of the electromagnetic spectrum (i.e., light having wavelengths of between roughly 0.4 and 0.7 μm in length).

Firing Controls

Initial tank guns were fired by percussion ignition, where a striker drove a firing pin mechanically into a sensitive primer, to initiate the firing sequence. With the initial tank guns being of naval (British) or artillery (French) origin, the strikers were actioned by pulling on lanyards. This subsequently gave way to firing switches that were either actioned by a foot pedal or a hand (finger) switch. These, in turn, would action an electrical solenoid to drive or release a firing pin for percussion ignition or to pass a current through a firing pin contact to an electrically-activated primer. These later are now used almost exclusively in modern tanks, although up to and including the Second World War, most systems were percussion ignition.

In addition to the actual firing controls, which were available for the gunner only, or to the gunner and commander, there was also usually a safety switch that required positive action before the gun could be fired. This was usually actioned by the loader (when there was one) to ensure that there was no hazard when the gun recoiled after firing.

Conclusion

This chapter has been written to provide an overview of tank guns and their components and associated systems. All these will be addressed in greater detail when looking at specific gun systems.

Bibliography

Kinnear, J. and Sewell, S.L., Soviet T-10 Heavy Tank and Variants, Osprey Publishing, 2017. ISBN: 978-1-4728-2051-8.

Krier, H. and Summerfield, M. (Eds), Interior Ballistics of Guns, American Institute of Aeronautics and Astronautics, 1979. ISBN: 0-915928-32-9.

Stiefel, L., Gun Propulsion Technology, American Institute of Aeronautics and Astronautics, 1988. ISBN: 0-930403-20-7.

The Royal Military College of Science, The Handbook of Indirect Fire Weapon Systems, British Ministry of Defence, 1982.

Chapter 3

Ammunition Aspects of Gun Launch

Introduction

This chapter looks at the internal ballistics of gun launch, as well as the various components of munitions required to achieve this, e.g., primers, igniters, casings, propellant and aspects of the design of a round of ammunition that are specifically to address the round’s interaction with the chamber and barrel.

A gun system can be considered as a projectile-throwing device, consisting of a projectile guiding tube (barrel) to which is connected a reaction chamber (chamber). The combustion (burning) of a solid propellant in the chamber transforms the solid to gases and results in the release of chemical bond energy, which appears as heat. These hot gases are confined by both the chamber and the round or warhead that is still in the barrel. This results in a very rapid pressure rise in the chamber, behind the warhead or projectile, with the consequence that the projectile is propelled down the barrel at a high velocity.

More specifically, upon ignition of the propellant charge, hot gases are evolved from the burning surface of each grain of propellant. The pressure rise in the chamber proceeds rapidly. Due to the initial high resistance to projectile motion, high pressures are developed in the chamber.

Eventually, the projectile begins to move down the barrel, effectively enlarging the chamber volume, which is accompanied by a pressure drop in the chamber. However, since the burning rate of the propellant charge is directly proportional to the chamber pressure, the initial pressure in the chamber increases the burning rate. The net result is that a point of maximum pressure, also known as peak pressure, is reached, usually after the projectile has moved a short distance down the barrel.

Beyond the point of peak pressure, the chamber pressure begins to drop. By the time the projectile reaches the muzzle, the chamber pressure will have been reduced to about 10–30 per cent of the maximum. A barrel sufficiently long to allow the chamber pressure to reach atmospheric pressure would be much too long to be practical.

Pressure at the muzzle is dependent on the design of the system being used, as well as the type and quantity of propellant charge used. The magnitude of the muzzle pressure is important, as this pressure continues to work on the projectile for a short distance beyond the muzzle. Thus, the projectile continues to accelerate beyond its exit from the gun.

For gun systems whose projectiles are to be spin-stabilized, the bore is rifled. Rifling involves the engraving of a set of spiral grooves into the bore surface, which is made up of lands (the raised portions in the barrel) and grooves, and acts as a guide in causing the projectile to spin. Rotational velocity of the projectile will vary directly with the angle (tangent) of the developed rifling curve. Rifling is often specified by a characteristic parameter called twist, which is expressed in terms of calibres per turn, or the bore length measured in calibres in which the rifling makes one complete turn. Twist can be either uniform (constant angle of rifling from bore to muzzle) or increasing (gain), where the angle varies in accordance with an exponential curve. An example of rifling can be seen in Figure 2-4.

Projectile

Projectile design is dependent on the type of target to be attacked. Obviously, an important parameter is the mass of the projectile, as this affects not only the velocity and acceleration of the projectile but also the rate of pressure rise during the early part of the ballistic cycle. These factors will, in turn, influence the velocity at which the projectile leaves the barrel (the muzzle velocity), as well as the kinetic energy (energy of motion) able to be delivered to the target and the range to which the projectile can be delivered.

Typically, a projectile is designed for aerodynamic stability and minimum drag. For larger guns, the shape of the forward end of the projectile found to give the smallest amount of air resistance is the ogive, which is defined as an arc whose centre is on a line perpendicular to the axis and whose radius is expressed in calibres. Typical values of the radius are seven to nine times the diameter of the projectile. For smaller projectiles, the ogive shape often is replaced by a cone. Also, the head of projectiles may be made blunter to give more effective penetration and to make the nose less fragile.

Both the rifling of the barrel and the rotating band of the projectile are used to impart and transmit torque (turning force) to the projectile. The rotating band is usually made of soft metal, which is securely sealed to the projectile body. The forward edge of the rotating band is tapered slightly to facilitate the engagement of rifling. At this point (before moving down the bore) the rotating band must be firmly mated to the bore surface and engraved or cut into by the rifling. This engraving process introduces or causes an initial large starting pressure. Ideally, the engraving would delay projectile travel until the burning rate and gas pressure were at optimum values.

In general, the internal ballistics of a particular gun is affected by variations in the physical and chemical character of the gun propellant.

Complete Round

A round of ammunition contains or is composed of two major components: the projectile and the propellant system. The projectile may have a solid inert core (for armour penetration) or be a shell containing high explosive (HE) or special filling, e.g., pyrotechnics. The projectile may also have a jacket, a driving/rotating band, discarding sabots, a tracer and a fuze.

The propellant system will have the chemical propellant within an inert (metal) casing, a primer to ignite the propellant and perhaps an igniter to facilitate the ignition of the propellant.

Fixed Rounds

Typical fixed rounds are shown below in Figure 3-1. The term ‘fixed’ denotes that the round is a complete entity in that the projectile is firmly attached to the casing. This type can be found for up to 90 mm tank rounds. Fixed rounds are fired from guns with sliding breeches that are called by the British QF, or quick fire. The obturation (breech sealing) in this case comes from the expansion of a cartridge case. Examples of a sliding breech are shown in Figures 3-2 and 3-3.

Separated Rounds

As tank guns and their ammunition became larger, in terms of both weight and length, it became necessary in most cases to separate the projectile from the propellant casing. This became the practice for Soviet and Russian guns of 125 mm calibre, which first appeared in the early 1970s.

Western post-war guns based on the German 120 mm cannon retained fixed rounds. The only case of separated rounds discussed here was for the Soviet 122 mm D-25T L/43 gun mounted on the JS-2 and JS-3 heavy tanks (as well as JSU-122 tank destroyers). The cartridge cases were rigid, so a sliding breech was still possible, although the projectile and cartridge case were loaded separately. Drawings of these rounds can be seen in Figure 3-4.

Figure 3-1 Photo of the Pzgr Patr 39 (APCBC) rounds used in the 88 mm KwK36 L/56 (Tiger I) on the left and the 88 mm KwK43 (Tiger II) on the right (same projectiles). (blog.tiger-tank.com)

Figure 3-2 Drawing of the horizontal sliding breech of the 76.2 mm Ordnance QF 17 pounder gun in the closed position, with the breech block within the breech ring. The breech opened to the right, with loading performed from the left side of the gun. (Tank Museum Archives, Bovington, UK)

Figure 3-3 Drawings of the vertically opening breech open (left) and closed (right) of the British Ordnance QF 75 mm gun, as used on the Churchill VII and Cromwell tanks. (Tank Museum Archives, Bovington, UK)

Figure 3-4 Drawings of 122 × 785R mm rounds fired by the D-25T tank gun on the Soviet JS-2 and JS-3 tanks. Projectiles from left to right are OF-471 HE, BR-471 APHE and BR-471B APC. (military.wikia.org)

Smoothbore Guns and Ammunition

The discussion above has dealt with rifled guns and the ammunition they are typically used to fire. After the Second World War, however, gun systems used fin stabilization for the round in flight, so no (or very little) spin is to be imparted to the round by the barrel. Consequently, the barrels have no rifling and are called ‘smoothbore’. As tank guns in use through 1945 were all rifled, there will be no further discussion of smoothbore guns or ammunition here.

Ignition Train

Gun rounds are launched primarily by the conversion of solid propellants into hot gases, which in turn exert a high pressure on the base of the round while it is still in the barrel. These propellants, though, are designed to be high in energy content but relatively insensitive. By this is meant that gun propellants are designed to not respond to minor stimuli, or even to small arms rounds that might strike them. Consequently, it is difficult to initiate the combustion process that converts the propellant solid to gases. To compensate for this, an initiation or ignition train is used that starts with a much more sensitive but quite small amount of material, called a primer. This is known as a primary explosive, and it, along with gun round propellants (secondary explosives), is designed to burn quickly, or deflagrate, not detonate. The difference between these two is that for a deflagration, the burning or combustion front travels through the material at a velocity below the speed of sound (sonic velocity) in the material, while for a detonation (or explosion), the combustion front (detonation wave) travels supersonically.

For larger shells, with a significant amount of propellant (a large propellant bed), there is often an intermediary substance that is placed between the primer and the propellant, often called an initiator, booster or igniter (although the terms primer, igniter and initiator are often used interchangeably, to some confusion). The quantity of initiator is often much larger than the primer, while it is usually less sensitive. Also, the initiator will often extend into the propellant bed and is designed to ignite the propellant as uniformly as possible (primarily to avoid pressure fluctuations within the gun chamber).

In general, then, gun rounds start the initiation train with a small quantity of sensitive primer, which will, in turn, either ignite an intermediate initiator of greater quantity but less sensitivity, or directly ignite the insensitive propellant. These components will all be examined in greater detail below.

Primers

Primers (sometimes called percussion caps) fall into two main categories: percussion primers that are initiated by a mechanical stimulus, such as being struck by a firing pin, and electrical primers that are initiated by electrical energy being passed through a wire in the solid primer composition. Both types involve the application of energy to the primer.

The basic percussion primer element consists of a heterogeneous mixture of energetic components which is initiated by a mechanical energy source in the form of a moving firing pin.

A ductile metal cup is used to contain the energetic pellet, which is held in position by a coated paper disc and anvil combination.

The primer is assembled by loading the impact-sensitive mix into the primer cup, covering it with a paper disc, consolidating the mix and then pressing the anvil into place. Initiation involves impacting the primer cup with a hemi-spherically tipped firing pin with enough kinetic energy to compress the pellet mix against the anvil, causing deflagration. The ductile primer cup is usually not penetrated by the firing pin and is capable of maintaining a high pressure gas seal on the input side. This causes the combustion products to burn through the paper, travel around the anvil legs and through an exit vent to the next element in the initiation train. An example is shown in Figure 3-5.

Figure 3-5 Basic percussion primer element. (Krier and Summerfield, 1979)

Most tank guns described here used percussion ignition, although German tank guns fielded during the Second World War used electrical ignition (as do modern tank cannon ammunition). Electrical ignition involves passing a small current through a primer element at the base of the round.

Igniters

As mentioned above, larger shells use igniters as intermediate stages in the ignition train, between the primer and the propellant. In these cases, the igniter is the principal energy