Well I guess there were a couple of different versions of the M16 MGMC.. the M16, A1, and A2.
The regular M16 was a purpose built AA vehicle, but the A1 and A2 were regular M3 personel carriers converted for AA purposes.
I am not sure which is which, but I think the regular AA-built M16 looked like this:
It had flip down armored sides, so the gun could rotate 360°. I assume they flipped them up when travelling etc.
The A1 and A2 came from regular M3's so they didn't have the flip down sides, I don't know if they mounted the Quad-.50 higher or what. I've seen some like this:
Some with notches carved out, and some with the flip down sides installed.
Looks like the 21st/JSI one is specifically named the M16A1 MGMC.. and it has the rear door, which is correct for the A1, but for some reason they didn't do whatever was needed to get the gun to actually MOVE.
The Bravo Team one is clearly a standard M16:
It has the flip down sides AND notch (sides do NOT flip down, but I assume the notch lets it rotate), but is missing the rear door access and stowage racks. I also don't think the little window covers move. On the 21st/JSI they all move/flip up/open & close etc.