Don't expect anything realistic. Either the author doesn't understand firearm law in America or they purposely wanted to limit the power of the MC.
At one point in Vol. 4 they're going to go to a tavern where the guy trafficking elves is supposedly going to be and he asks the goddess for a handgun. A Glock 17, Walther P39, anything. Goddess flips through their store's catalog and says, "Nope. We don't have anything like that. You need a license to sell handguns that we don't have (uh, bullshit, that's not how that works). So the only firearm that the protagonist buys so far after that is ANOTHER Remington 870 that he just takes a hacksaw to and makes a sawed-off whippet gun out of.
I'd be shocked if he got even a semi-auto shotgun or rifle at this point. So count suppressors or anything fun or realistic that could actually buy in America likely out of the question.
Although, as shown with the 870 he chopped, the BATFE can't chase you to another world. So violate the NFA all day ever day. Feds can't do shit.