Of all the AR-15 gas-system setups on the market, the carbine-length system is one of the most popular and widely seen. This is due to the fact that carbine-length gas systems were found on the military's M4 carbine models, and that fact translated over to commercial copies of the gun. This CMMG-made buffer is designed for use in carbine-length systems. Simply pair the carbine buffer with a compatible carbine-length spring, and you'll have a receiver extension designed to run reliably with your carbine-length gas system.