I have been looking for a photo of an M9 Bayonet mounted on a Midlength Carbine to show how these nount for a long time as from my own measurements I thought that the bayonet wouldn't quite mount correctly with the mounting ring over the open part of the flash hider...
Well, after looking for a couple years I have finally had someone e-mail me a couple photos:
Colt 14.5-inch M4 Barrel with LanCay M9 Bayonet
Rock River Arms 16-inch Midlength Barrel with LanCay M9 Bayonet
Note in the top photo how the Bayonet mounting ring fits over the flash hider where the flash hider that is threaded to the barrel. In the second photo the mounting ring is actually forward of the barrel.
Personally, I find this to be a foolish mistake of Armalite back when they came up with the specifications for the mid-length upper. Then, RRA was foolish enough to use Armalite's flawed lead!
HUGE THANKS TO CHRIS FOR SENDING US THESE IMAGES!