From Wikipedia:
M1 is: a tank, a bayonet, a carbine, a chemical mine, a light tank, a rifle, a mortar, millitary designation of Thompson SMG, an underwater pistol, a 120mm AA gun, and a shotgun (M1 Super 90)

M2 is: amunition, an AFV, a carbine, a machine gun, an armoured car, a half-track, a howitzer, two kinds of tank, a mine and a flamethrower.

M3 is: an SMG,a carbine, an anti-tank gun, a fighting knife, a howitzer, and a shotgun. (M3 Super 90), and 6 kinds of AFV/tank.

M4 is: a carbine a tank, an AT gun and some other stuff.

M16 is the rifle, a mine and a varient of the M3 half-track with a quad .50cal MG turret for AA

And so on. M1 must mean the first modol, since most of the M1 and M2 suff was US WW2-era equipment. So you could drive an M1 tank, with the crew all having an M1 rifle, carbine, shotgun, and SMG, all with differant ammo. Then an M1 AA guns takes you out, so you dig in for the night and use M1 mines to protect your position before crossing a river armed with an M1 underwater pistol. Confused yet? Shall I go on the M2 and M3?

Don't forget about the Benelli M4, and the Spectre M4.

Ah, but the Spectre M4 and Benelli M4 are commercial designations. The Benelli is the M1014 in US service, and the Spectre is not a US service weapon. Neither is the Benelli M1 - in fact, there was never an M1 shotgun. Until the Benelli, shotguns in US service were just known by their commercial names.

I believe the military added the "M" at the beginning, though. Every time I've seen or heard the Remington 870 or Mossberg 590 mentioned, they're only referred to as "M870" or "M590" when referring to US military use.

I could be wrong, though.