Probably the Russian built PTRD-1941 anti tank rifle in 14.5 mm.
you will have to define if you mean commercial or wildcat.
Yes
The 50 caliber pistol is the highest pistol which can be purchased without the need for a permit. Some pistol shotguns exist which can be 75 caliber weapons.
Unknown.
Stevens.
Remington
Take the rifle to a gun shop so they can figure out what make, model and caliber rifle you have. There is no 84 caliber long rifle.
Springfield Armory
H&a
They were made in several calibers- will be marked on the barrel.
The .950 JD Jones is the largest sporting rifle caliber made.
WHICH rifle? There is more than one type made in this caliber.