Buffer Tube

A misnomer, but common-use synonym for “lower-receiver extension,” a buffer tube is a component found on the original design of the AR-15/M4 platforms. The buffer tube contains the buffer and buffer spring. These components slow the rearward travel of the bolt while building kinetic energy to return the bolt forward, stripping a round from the magazine, chambering the round, then closing the bolt and locking the chamber. It is the area to which a butt-stock or brace is attached.