>M1 Abrams tank
>M1 bayonet, as fitted to the M1 Garand
>M1 carbine
>M1 Combat Car, not to be confused with
>M1 Armored car, which is not the above tank
>M1 Garand, a battle rifle, onto which one would fit said bayonet, and
>which isn't the carbine
>M1 Helmet
>M1 mortar
>M1 rocket launcher, a bazooka variant
>M1 Thompson submachine gun
>Mk 1 Underwater Defense Gun, an underwater pistol
>M1 Thompson carbine, which isn't the submachine gun
>57 mm Gun M1, also known as the British Ordnance QF 6-pounder
>90mm M1 Gun, one of a family of dual purpose guns and not the above
>6-pounder
>120 mm M1 gun, an anti-aircraft gun and not the 90mm. OR the 6-pounder
>240 mm howitzer M1, see previous bullets
>M1 Super 90, a semi-automatic shotgun made by Benelli
>HMS M1, an early submarine, noted for being equipped with a
>capital-calibre gun
>M115 howitzer, also known as the "8-in Howitzer M1"
>M1 flamethrower
>
>There's a handful of others which needs a bit more of a stretch to
>'fit' under the M1 name, but...

Wikipedia has, of course, an excellent disambiguation page.

Notable other entries in that list are the mustard-gas landmine, the Crab Nebula, and the motorway between London and Leeds.