If I recall correctly - and this is just off the top of my head - under US FEDERAL law (and I stress federal: states and localities can and do vary tremendously), cartridge guns made before 1898 are automatically antiques, and unregulated, regardless of caiber.

There are also guns of later manufacture that are in calibers designated obsolete, that are not regulated (though I think you might need a "curios and relics" collector's license to be able to circumvent normal federal regs on purchasing them: but I'm not sure abut that).

What are the corresponding UK regs?

Jim