An automatic firearm that includes a unitary portion including a human interface and a barrel defining a longitudinal dimension. A slidable portion is attached to the unitary portion so as to permit the slidable portion to slide in the longitudinal dimension. The slidable portion includes at least a portion of a firing chamber. In addition, a lock is adapted to selectably fix the slidable portion relative to the unitary portion. Also, a chamber attached to the unitary portion and an inertial member permitted to move inside the chamber and urged into a first position, the inertial member adapted to translate into a second position, displaced from said first position by at least 0.1 cm, upon firing of the firearm, in which second position the inertial member begins to change the position of the lock, thereby permitting the moveable portion to move relative to the unitary portion.