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.
