Guys, I think I'm just an idiot, sorry for wasting your time. I looked a little closer, and it seems that it's not the flash hider.
I think it may be that the bayo lug may be to blame.
It fits just fine on my 20" A2 just fine.
From what it looks like, the fit between the bayonet and the lug is really tight. So maybe it's just the fit. BTW, the upper is brand new.
So, should I buff the inside of the bayonet where it locks so it will go on easier? I've had to do this with AK bayonets before, but wonder if I should just try out another bayonet.
Thanks for the help by the way.