Can you get 84mm Charlie-G recoilless rifle ammunition in the US? That is quite effective ammunition from the late 1940s!
Unfortunately, we are limited to .50 caliber here (for cartridge ammo, black powder projectiles and shotgun slugs can go larger). .50 cal is a little over 12.5mm. So I would think that 84mm cartridges would be right out. Bummer!