Two things:
1) The M1 Carbine loads from a detachable magazine, no thumb involved
2) The only time the Garand bites your thumb is if you haven't maintained it properly and you load it wrong. If the rifle is maintained properly the bolt hold open will hold the bolt back after the enbloc is inserted, and then will release the bolt forward after you cycle the charging handle. Even if the rifle isn't maintained properly it still won't bite you if you hold your hand on it properly. The proper way to hold your hand on the rifle is to put your right thumb on top of the enbloc while keeping your palm against the right side of the receiver so that the meat of your palm is against the charging handle, this way if the bolt does jump forward on its own it still won't bite you because your palm is in the way.
Because the teeny tiny amount of explosive filler you'd get in most rifle cartridges, even for a full sized rifle cartridge, wouldn't be enough to actually do anything. Plus body armor plates whether they be ceramic or steel are covered in spall liner that prevents HESH from working effectively anyways. Plus the lower density of the projectiles would (in all likelihood) adversely affect accuracy. Plus you'd get a lot more mileage out of just using a hard, dense material such as tungsten or cobalt as a penetrator. Plus the easiest way to get around body armor is to simply teach your guys to shoot for a belt buckle hold anyways.