It wasn’t so long ago that 9mm ARs were little more than range toys — novelty items that were kinda fun when they ran, but not particularly reliable and somewhat crude. Even Colt’s version didn’t have a stellar reputation, despite being designed around the double stack, two position-feed Uzi submachine gun magazine. The first company to really crack the code when it came to getting a blowback AR to run consistently was JP Rifles, and their combination of dedicated 9mm bolt carrier and silent captured spring system allowed users to quickly tune the gun to their ammo. Further refinements evolved, including a short stroke kit that reduced bolt throw and recoil, but the basic physics of using mass and inertia to balance the rearward thrust of a 9mm projectile remained.
Because Newton’s third law is a bitch, this leaves users combating the impact of a pound and a half of reciprocating mass, which ironically makes blowback 9mm ARs recoil more than their 5.56 counterparts, despite generating less than
