Shopping for an AR-15 barrel or complete upper means running into the terms pistol, carbine, mid-length, and rifle gas system. They look like simple labels, but gas length directly affects recoil, reliability, and how well a rifle runs with a given barrel and caliber. This guide breaks down each length so you can choose the right setup for your build. What Does the Gas System Do? The standard AR-15 uses direct impingement. When you fire, the bullet passes a small hole in the barrel called the g …