for a rifle it is thought to be a .50 calibre but a gun company made there own for there own sniper rifle the .408 calibre
For a rifle it would be the 2 Bore.
Smaller than 20mm.
The .243. It's basically a necked down .308 Winchester.
The largest calibre i came across was 30 calibre russian rifle (gas chambers punctured to make it bolt action). owned by a retired army personnel. I guess they allow them to purchase it from the ordnance. Don't know if they can sell it though.
This all depends on the calibre of said rifle make and model :)
The Winchester Company introduced the 32 Calibre rifle in 1873. For it they created four different types of bullets including the Winchester 32 special.
It's Japanese, and the rifle uses the 7.7x58mm Arisaka cartridge.
No
It's a rifle, in fact very small calibre. .22 is roughly 5mm's.
A 243 calibre rifle is Just that. The bore (internal measurement of the barrel) is 0.243 of an inch in diameter.
303