The firing pin hits the primer on the rifle cartridge.The AR-15 uses a direct-impingement system with a gas tube to cycle the bolt: The spent casing is ejected, the bolt drives forward, and a new round is chambered.The kinetic energy of the spent casing drives the bolt back into the buffer tube.That same gas forces the spent casing to slam against the bolt inside the chamber.The rapid expansion of gas expels the bullet from the barrel.Powder burns in the firing chamber and creates gas.The firing pin hits the primer on the 9mm cartridge.That’s because the AR9 usesīlowback to cycle the bolt. Look closely at a typical AR9 upper, and you won’t see a gas tube anywhere. There’s a lot of discussion – and some confusion – about how these two guns work. The AR9 and AR-15 look alike, but each uses different parts and systems to operate. Since the AR9 pistol and AR-15 appear so similar, let’s compare the two first. A 9mm-specific buffer is required inside your buffer tube, but we'll touch on that later. Converted 5.56/.223 guns use special parts to adapt the factory upper and lower receivers for the 9mm cartridge. The AR9 pistol instead uses a new lower receiver (aptly called theĪR9 lower) and a stripped AR-15 upper receiver with a 9mm barrel, new bolt carrier group, and an AR-15 buffer system and lower parts kit. But this particular 9mm weapon is not just an AR-15 with a 9mm Parabellum conversion kit installed. The AR9 pistol (sometimes hyphenated "AR-9") is a new 9mm variant of the AR-15. All data and recommendations in this guide were compiled from real shooters who've built and tested AR9s with different parts.Ī 9mm AR Pistol Kit (which must be paired with a 9mm AR lower) is what many shooters buy to build from scratch. Looking for answers about shouldering an AR pistol brace? We wrote a guide for that, too. When we're through, you'll know everything you need to build the perfect AR9 pistol in a supersonic or subsonic (suppressor-friendly) configuration. This is your ultimate AR9 pistol guide for 2020/21 and beyond! We’re breaking downĮverything you need to know to build one yourself: What the AR9 pistol is, what separates it from an AR-15, parts compatibility, 9mm Parabellum's ballistics, buffer weight, barrel length, twist rate, and magazines. Updated September 2020 with new ballistic chart.