This AR-15 buffer tube length guide explains the major categories at a general level without providing assembly instructions or performance-tuning advice. Understanding the Purpose of a Buffer TubeThe buffer tube, also called a receiver extension, is a structural component positioned behind the firearm’s lower receiver. The term buffer tube for AR-15 builds is often used broadly, but several distinct product families exist. A dependable AR-15 buffer tube length guide should emphasize the relationship between the receiver extension, stock, recoil components, receiver pattern, and manufacturer specifications. A buffer tube for AR-15 builds is not merely a cosmetic stock-mounting accessory.