Lock back check tests whether or not the rifle locks back on the last round. Place one round in the mag, load the rifle and shoot it. If it doesn't lock back, it's short stroking. When tuning with an adjustable gas block, close down the gas until the rifle ejects but does not lock back. Then open it one click at a time until it ejects and locks back.

If the carrier is moving too far, the buffer will hit the back of the RE and recoil will feel sharp. Sometimes it will sting the face of the shooter.