Large caliber here would mean artillery size, maybe mortar or grenade launcher. None of that is commonly consumed by civilians. There are routes to get there,

Large caliber here would mean artillery size, maybe mortar or grenade launcher. None of that is commonly consumed by civilians. There are routes to get there, but it seemed difficult to me (I looked once upon a time). The largest caliber even somewhat commonly used by civilians is .50 BMG (12.7x99mm NATO). I have seen 20mm rifles and ammo for sale (once), but it is a HUGE pain in the ass to get them and probably rightfully so. Anything over .50 is automatically considered a destructive device by the ATF unless a specific sporting exemption is given to it (that's generally only for big game rifles). Getting one of those requires a fistful of certifications last I checked; I think you effectively have to be an arms dealer to get one (or at least pretending enough to be certified as one). None of this is legal advice, talk to the ATF and/or a lawyer before trying to buy anything too exotic.