This wondrous item is a gun barrel made out of ice, though it doesn't seem to melt and is still cold to the touch. A frosty mist emits from the barrels sides, almost like vents.

Freezing Point: When you make an attack roll against a creature you can expend a charge to attempt to freeze it. If you hit the creature makes a DC 12 Constitution save. On a failed save the creature suffers the Petrified. The barrel has 1 charge and it recharges after a long rest. On a successful save the creature feels a bit chilling. The target becomes unpetrified after 1 minute. 

Cold Nature: The weapons damage changes to cold.

Firearm Modification: This barrel is meant to be an attachment for firearms. When attached to a non magical firearm it will become +1. If it is attached to a magical firearm then it will attain no bonus. Note, only one barrel attachment can be added to a firearm unless it has multiple barrels.