The AR-15 gas block sits over the barrel’s gas port and routes propellant gas from the barrel back into the action. It connects to the gas tube and drives the bolt carrier group rearward to cycle the action. Builders choose between low-profile, adjustable, and fixed front sight base styles for different upper builds. It fits carbine, mid-length, and rifle-length gas systems on standard AR-15 barrels. An AR-15 gas block works with handguards, gas tubes, and bolt carrier groups to keep the cycling system reliable.