In some embodiments, a firearm comprises a lower receiver, a bolt carrier and a weight. The lower receiver is arranged to support a hammer and an auto-sear. The bolt carrier comprises a body and a striker moveable with respect to the body between a first position and a second position. The body defines a first cavity adjacent the striker. A weight is positioned in the first cavity and arranged to move the striker to the second position, wherein the striker is arranged to operate the auto-sear.