The caliber of the firearm should be stamped on the barrel.
It depends on what kind of ammo.
the m16 uses 5.56 caliber ammo
what kind of pistol is it?
It takes all your reserve ammo in your gun. Even If a zombie touches it, it still takes your ammo
almost every kind of 7.62x39mm
Lincoln's Repeater uses .44 Magnum ammo.
22LR
9mm rounds.
30/06
.380 ACP
38 special.
It will be printed on the barrel