Royal Ordnance actually had two designs for the XM9. The first, as shown on the left in the photo, is basically identical to the British SA80. It order for this bayonet to work on the M16 the flash hider has to be either changed or modified. The SA80 indexes on the rifle by locking into a small groove milled into the bottom of the flash hider. Without this groove the bayonet will just spin on the barrel. Royal Ordnance realized that the chances of the US Army changing the flash hiders on all of the M16s in service was slim to none, so they came up with an alternate design. The new design has a conventional muzzle ring and pommel with a mortise slot. This bayonet is shown on the right in the picture.
