Yesterday I received my Phrobis M9 bayonet from Midway USA. One of the first things I tried was to mount it onto my Colt 14.5" LE 6920. The locking mechanism slid on to the bayonet lug halfway but would not go any farther. I cleaned the small amount of dirt out of the locking mechanism on the bayonet and it still would not move. The locking mechanism opens and closes all the way. I have removed the locking mechanism from the bayonet to try to clean it better and take a micrometers to the bayonet lug and the locking mechanism and see if one is slightly to bis and maybe needs to be filed down a hair.

Does anyone have any suggestions on what could be stopping the 2 pieces from going together? Thanks in advance for the help.

I found the problem. The barrel is a 14.5" with a pinned and welded extended A2 flash hider. The spot were the pin is welded over is just high enough that it doesn't allow the bayonet ring to go past it.