Technically most 50 caliber rifles are .510", even the 50 caliber handguns are .501"
but then the ATF nixed Taurus's 28 ga judge offering, so you never know... I guess I wouldn't expect much flexibility right now in the USA... if you could start in Africa hunting big game, 1st ( not going to happen with anything I do )
also a detriment might be if you were starting with a 20mm military barrel, & trying to build a sporting rifle, you might not get too far ???
In life you either make dust or eat dust...