Mister Freeze's freeze gun was Mister Freeze's primary weapon which could instantly lower the temperature of its target, freezing it solid. His expertise with the gun was such that he could also use it to freeze ambient moisture, generating ice out of thin air for both offensive and defensive purposes.
After an accident with cryogenic freezing tanks rendered Victor Fries unable to survive outside of a sub-zero environment and became the tragic villain known as "Mister Freeze" after using his scientific genius as an expert in cryogenics to create both a vacuum tight suit which maintained his body temperature at 50 degrees below zero (while tripling his strength) and a laser-powered freeze gun. When Freeze sought revenge on his former employer Ferris Boyle for causing the cryogenic accident and seemingly doomed his wife. Freeze and his henchmen committed a series of robberies in order to build a freeze cannon (a larger-scale version of his freeze gun). Upon its completion, Freeze used the cannon to seal off the GothCorp building during a ceremony where Boyle was to be named "Gotham Humanitarian of the Year". Freeze attacked the room and used his freeze gun to encase Boyle in ice up to his waist, intending to kill the GothCorp CEO slowly but Batman arrived on the scene and stopped Freeze by using a container of chicken soup (provided by Alfred Pennyworth) to crack Freeze's helmet, compromising his suit.
When entrepreneur/businessman Grant Walker broke Mister Freeze out of prison to gain immortality through Freeze's own condition, he provided the former scientist with a new environmental suit and freeze gun. After being made "immortal", Walker revealed his own version of Freeze's freeze cannon, which he intended to use to destroy the rest of the earth save for the people under his watch. Realizing Walker was a madman, Freeze ultimately betrayed Walker and helped Batman and Robin evacuate and demolish Walker's "Utopian" society.