The issue of putting a bayo on a pre-ban mid length (Armalite or RRA) has come up on many threads.  I'm distilling it down a little, and seeing if there is an answer.  I have borrowed liberally from many posters here.

1) The mid length lug is closer to the end of the barrel than on a standard 14.5" or 20".  Some have posted 11/16", but I'm unsure of exactly what or how that was measured.

2) When a bayo is mounted, the mount does not go around the rear ring of a standard A2 fs.  It sits somewhere on the slots.  Some have posted that it bisects them, someone else posted that it just touches the rear of them.  Add to this the fact that the slots on an A2 have 2 points that could be considered their beginning (the initial point of relief and the point where it is open all the way through) and there is more uncertainty.  

3)  In response to a request for measurement, Chuck states
"OK, here are my measurements from aft end to where the slot meets the bore.

These distances will vary depending on how tight you screw the device on the muzzle. The M16A2 compensator only indexes on a full thread, all the others are non-indexable and can be set deeper or shallower. I recall the thread pitch is 1/24 inch (but someone will know for sure!).

M16A2 (no bottom slot): .75"
Vortex: .82"

Note the slots on the Vortex and Phantom are straight cut from outside to the bore, the USGI slots are tapered so the outside of the slot is longer than the inside. Measurements were made to the edge of the inside of the slot"

4) A vortex may allow a proper mount (or at least on that does not intefere with any gas coming out of the ports.  

SO ...

Does anyone have a mid length with a vortex on it who could report on the fit?  If not, can anyone with an A2 do the comparison?  

The distance from the edge on the bayonet lug where the butt end of the bayonet handle touches forward to the front of the bayonet guard is 5.08"  Call it 5.1".  

An easier way to measure if you don't have a lug:  The rear section of the bayonet guard, the part with the hole for the barrel, is 4.9" in front of the front sight forging.

The bayonet will "fit" on the rifle -- for length anyway -- even if there's no muzzle device.  A sloppy fit, but it won't fall off.

-- Chuck