Largest Caliber Cartridges
Re: Big, I say BIIGGG bore rifles.
Yeah, generally speaking, anything over .50 caliber is considered a "destructive device," and subject to BATF regulations. Certain collectors items, such as the .577 and .600 Nitro Express are okay, because these have little value as shooters and ammo is very difficult to locate. There's even a .700 Nitro - - I guess this is the one you mention in your question - - which was (briefly) made in recent years, just for the "I got the biggest gun ever" boastful collectors market. I think there was a short run of ammo made, at $100 per cartridge. The price is certain to have increased by now. (I read of the .700 about seven or eight years back.)
Then there were the old black powder heffalump pounders like the Four Gage. Hard to imagine a cartridge gun using a round ball that weighed four ounces, and a cylindro-conical bullet weighing more like SIX ounces!
Check around at the gun shows--the ones where there is a lot of collector interest-- and you'll find you can buy a decent surplus rifle for the cost of a single round of some of the big bore ammo. And a nice car for the price of the big bore rifle to shoot that collectors ammo.
We're not talking about "destructive devices" that an outlaw would want to blast open an armored car or a bank vault. You CAN buy these legally in the USA, but I'll bet there's not many bought or sold.
Best regards,
Johnny