I am pretty confident that this was either a huge screw-up OR Armalite did this so that is some idiot went out and used a bayonet to commit murder that Armaite would be able to say... "It wasn't designed to be used with a bayonet!"
Armalite is a stand up company so I have a feeling it was just a screw-up...
That difference in less then an inch would not impact the function of the weapon at all... In fact, I would rather see Armalite or RRA make the barrel just a little longer (16.6-inches or so) such that the bayonet would mount corectly...
Also, it is funny how Knights Armament Medium Short FF RAS Tube is the correct length so that if the gas block was mounted just forward of the FF RAS that the bayonet would mount in the right location... Following is some details on why I say that and how I still plan to build a 300 Whisper Upper if I ever stop spending all my money on other projects:
quarterbore.com/300whisper/300upper.html
I have wondered for a LONG TIME if KAC was going to make some mid-length carbines to the CORRECT size but I have yet to see one where they did it!