The caliber of the firearm should be stamped on the barrel.
It takes all your reserve ammo in your gun. Even If a zombie touches it, it still takes your ammo
It depends on what kind of ammo.
the m16 uses 5.56 caliber ammo
what kind of pistol is it?
almost every kind of 7.62x39mm
Lincoln's Repeater uses .44 Magnum ammo.
9mm rounds.
22LR
30/06
.380 ACP
Sasme as the M16. 5.56x45mm
The kind that renders a target lifeless.