I'm pretty sure...
...that the ban is only on "military" cartridges in excess of .50 caliber. Examples being 20mm (Solothurn, Lahti), 14.5mm (Russian Heavy Machine Gun Round), etc. Civilian rounds in excess of .50 caliber, like .577 Tyrannosaur, .600 Nitro, and .700 Nitro Express, if you can find them, are legal, as far as I know.
(I didn't know .700 NE was a real cartridge...I referred to it once in a dream, only to find out later that it was real....weird.)