What do you term a BP gun? Cartridge would have to be sec 7 but m/l revolvers are sec 1.
You can have a smooth bore m/l shotgun on your SGC even firing single shot.
If you want an obsolete calibre you can have it as an antique if you want to fire it you would have to have it under sec 7.3. It would have to be stored at a club passed to cater for them such as Bisley and I dont think there are allowed to be any non sec 7.3 licensed persons at the range while you shoot.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane