Here's where I'm at on the m4m7 hybrid for a 11.5 SBR with a B. E. Meyers flash hider.
I used a m4 blade because it was slightly shorter and allowed me just enough room to cut down the rear of the tang and keep the factory way of attachment. I've still got to put one more weld on the pomel to keep everything togeather but I'm waiting to finish it until I get a FSB with a bayonet lug for my colt so I can verify the length before I do anything unreversable.
You start by removing the pomels the.
The guard is held in place by a piece of steel thats bent in a U shape. This will have to come off on both bayonets.
This little bit of length difference seems trivial but if you use the M7 blade you cant shorten the tang enough with out welding in the lightning cuts and reshaping it. It's hard to explain but the pictures will explain this. I felt this would be weak hints the M4 blade.
Slide the guard on the blade, this will later be secured with two small welds.
I've already cut some of the tang off so this looks missleading BUT this extra material will be cut off at slightly under the end of the pomel so I can fill it with weld when I'm done.
M4 bayonet blade, shortened m4 grips, m7 pomel and guard.