Armor piercing bullets are quite legal for long arms. They are only disallowed for public consumption in the handgun variety.
Before the recent scare, I could have pointed you to numerous in-stock, on-line sources for AP in .223, .308, .50, etc. Everyone seems to be out-of-stock right now...
Heck, even the CMP recently had oldish Greek, steel core .30-06 for sale.
Ah, didn’t know about long arms ammo.