"The Jackhammer is an automatic, gas-operated, 12 gauge shotgun which uses a pre-loaded rotating cylinder as its magazine. The cylinder has grooves incised on its outer surface which are engaged by a stud on an operating rod, so that as the rod oscillates it rotates the cylinder; it is very similar to the system of cylinder rotation used by the Webley-Fosbery automatic revolver. The Jackhammer barrel floats, and is driven forward by gas pressure after the shot is fired. It is then returned by a spring giving movement to the cylinder operating rod. The significant point about this movement of the barrel is that it disconnects the barrel from the gas-tight connection with the cylinder, allowing the cylinder to be revolved to index the next round, and then, on the return stroke, reseals barrel and cylinder together." (From Jane's Security and CO-IN Equipment 1994-95)
<img src="http://badmojojacket.homestead.com/files/bmj.gif" alt="Bad.Mojo: Born to Kill">