Serbu Firearms' upper receiver for the AR-15 platform uses a different operating system which eliminates the need for a buffer and buffer tube! The operating rod houses the main spring, and the gas system uses a fixed piston. The result is a gun that runs much cleaner, is more reliable, and can be more compact than any AR-15 which uses a buffer tube. The SU-15 fits any AR-15 lower receiver and uses standard AR-15 bolt heads, firing pins, cam pins, barrels and barrel extensions, eliminating worry over these highly-stressed parts. Parts commonality is also increased. Initially, we are offering the SU-15 in two receiver lengths and four different barrel choices. The standard receiver is 14.75" long while the short receiver is 12.55". Initial barrel choices will be as follows: 7.5" 5.56mm, 16" 5.56mm, 9" .300 BLK, 16" .300 BLK. The SU-15 can be configured as a pistol, but adding a buttstock to a version with a barrel under 16" without prior NFA registration is illegal. When updates are available on the SU-15, we will post the information on our Facebook page, so keep an eye out for any new info.