I made a few 16" middy barrels this past spring. With a standard 3 0z buffer they would cycle but not lock back the bolt with a .073" (#49) drill bit. They seemed to function fine with a #47 (.078") gas port but the range goes up to .085" for a 16" barrel, and I'd expect a shorter barrel would need at least that if not a little bigger. Pressure/volume/time needed to cycle. Make one leg shorter the other two have to compensate for it if they can.