Hmmmm, I think they would be C&R.
They are black powder weapons
Using not readily available non-cased ammunition
And while they are over .50cal, so is a .54 Hawkins, and you can get those mail order.
So I would think yes.
Wisconsin's main battery consisted of nine 16 in (406 mm)/50 cal Mark 7 guns, which could hurl 2,700 lb (1,200 kg) armor-piercing shells some 20 mi (32 km). The secondary battery consisted of 20 5 in (127 mm)/38 cal guns in ten twin turrets, which could fire at targets up to 10 mi (16 km) away.
Hmmmm, I think they would be C&R.
They are black powder weapons
Using not readily available non-cased ammunition
And while they are over .50cal, so is a .54 Hawkins, and you can get those mail order.
So I would think yes.
I was in Dahlgren when they test fired the last batch of 16" rifles before the BB's went back to sleep. The concussion liked to rip the fire door off the barracks and we were @ 1/2 mile behind the breech. Impressive....
They aren’t black powder or muzzle loaders nor antiques. So still DDs
Now an actual muzzle loader cannon is just fine. Cash and carry.
Something like an old Maine class pre-dreadnaught battleship main guns (12”?) should be perfectly legal even though breech loaded. Old enough and non-standard not readily available ammunition.
They most certainly are black powder. The 16" guns on the New Jersey class used black powder as a charge.
The D839 propellant (smokeless powder) grain used for full charges issued for this gun was 2 inches long (5.08 cm), 1 inch in diameter (2.54 cm) and had seven perforations, each 0.060 inches in diameter (0.152 cm) with a web thickness range of 0.193 to 0.197 inches (0.490 to 0.500 cm) between the perforations and the grain diameter. A maximum charge consists of six silk bags–hence the term bag gun–each filled with 110 pounds of propellant.
The red quilted primer patch on the end of each bag holds the black powder igniter.
The Navy used to use parts of APG as impact area for gunnery practice. I've found unexploded naval ordnance on Poole's Island as well as at least one unexploded 14" projectile on APG in the impact areas we still use. 14" guns were phased out before WWII if I remember correctly.
In Navy parlance they are "50 caliber".
The Navy refers to barrel length in calibers. (50 X 16" = 800" or 66 and 2/3 feet)
The USS New Jersey was an Iowa-class battleship. The Iowa-class battleships used the 16"/50 caliber Mark 7 – United States Naval Gun as the main armament.
This site has lots more information:
http://www.navweaps.com/Weapons/WNUS_16-50_mk7.php