I have to admit, I had no idea what an M26 MASS was....so I looked it up.
https://en.wikipedia.org/wiki/M26_Modular_Accessory_Shotgun_System
All I can say is, what a stupid freaking contraption that is! Looks super fragile like if you dropped it or banged it hard against something it would break or knock something out of whack.
Who would like to lug around such a contraption? How unwieldy do you think that thing is to handle? Who comes up with these brilliant ideas?
Stupid.