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