Sometime back I actually did acquire a National Postal Meter M1. Need to shoot it one of these days. The good bullets designed for it are not easy to find, however.
I think one outfit has now got an AR lower which can use grease gun magazines in .45 ACP, which are still fairly easy to buy. Combined with an appropriate .45 ACP upper, one would gain decent velocity and power, and still have a fairly handy weapon.
friend of mine locally owns a gun store. He had about ten boxes of m1carbine ammo, headstamped 1942, 30buck a box. I should have bought it all, as the carbine was one of my first collection addictions. You trip accross it but have to look, cmp sometimes sells it.
