You need to submit the kind of firearm is it? Pistol, rifle, shotgun?
6mm bb's
Personally I'd say the gun itself decides that haha, but generally speaking the 16-gauge is a good multi-purpose shotgun size, it (with the proper ammo i.e. slugs) can take down a deer no hassle.
Take it to a qualified gunsmith
Take it to a gunsmith.
9mm rounds.
How many cartridge does a 12 gauge shotgun carry.
almost every kind of 7.62x39mm
Lincoln's Repeater uses .44 Magnum ammo.
Take it to a gun smith.
Read the owner's manual or have a gunsmith do it.
You take it to a gunsmith