The AR-15 gas tube runs from the gas block to the upper receiver and carries propellant gas back into the action. It pushes the bolt carrier key rearward to cycle the bolt carrier group and eject the spent case. Builders select carbine, mid-length, or rifle-length tubes to match the gas system on their upper build. Each length requires a matching gas block port location on the barrel. An AR-15 gas tube works with gas blocks, handguards, and upper receivers to keep the cycling system running reliably.