That's interesting - - I never served in the military (I really kinda regret that in my older age, now) but saw the world via the Dept. of State, but I always thought that "M" meant military.
Maybe we can start a chain here, where folks can send in the "M" firearms the have known and used, with a short description. I would be glad to tabulate. It would be interesting if anyone knows the more obscure ones.
Steelhat (machinest, armorer student)