To put it simply, an AR9 is an AR-style rifle modified to fire 9mm Luger bullets. When you move away from the standard .223/5.56 chambering, you'll notice several changes. The AR9 is made up of caliber-specific components such as a barrel that is designed to fire a 9mm round, and a modified BCG (Bolt-Carrier Group). The AR-9 platform is known to have extremely reduced recoil due to the fact that it shoots a pistol caliber cartridge and holds above the average extra capacity of rounds in the magazine. They are also known to be extremely versatile and modular in the sense that OEM Glock magazines can be used, depending upon the build.