This why I put an APG Defense AR-15 Bayonet Lug Extender - Adapter on my 16" upper. Carbine length gas systems were made for 14.5" barrels since the early '70s, on the 651, 652, 653, and 654 M16A1 Colt Commando carbines, so when you put a bayonet on a 16" barrel the bayonet ring is 1 1/2" too far back. It's on the barrel instead of the flash hider, and the skinnier the barrel is, the more it flops around.