There are probably users who are more educated than me on the matter, but no gun store will ever sell you a firearm… or attachments…. Or rounds.
All states require firearms to be registered, and in order to register a firearm a customer must be a resident in said state. For example, if I want to buy a shotgun in Arizona, I'll not only have to be a United States citizen, but also a permanent resident of Arizona (permanent residence = shows as your home of record on legal documents). This is because each state has different requirements and laws regarding firearms, and what you can do with them.
As stated before, coming from the UK, or any other place, and trying to purchase a weapon as a tourist will simply be a waste of money. No gun store is gonna risk committing a felony to sell one to you because You purchasing, or even being in POSSESSION of a weapon here is 30 degrees of illegal.
Maybe you can get lucky off a private sale; where a private seller sells to private buyer, but I'd also not really spend the money for sure a big "if". Chances are you won't find anyone willing to legally sell you a weapon, and anybody who sells illegal weapons would likely just rob you if you met up with them.