The GEN12 was not the 1st ""AR shotgun"", but it is the first to use a standard 308 lower. The GEN12 upper was specifically designed to matte to any DPMS GEN1 308 Lower or other 308 lowers sharing the same pattern. The GEN12 upper will work with either the shorter DPMS buffer tube or the longer Armalite AR10A(R)/ 308 tubes. The center of the upper is the short recoil system which cycles the action. This means there is no gas tube or piston. The cyclic rate can be tuned by adjusting buffer weights and springs by the end user. The upper will cycle birdshot or magnums and most everything in between. It reliably feeds both 2 A 3/4" shells and 3" Mfg: Genesis Arms