Mister Freeze's freeze gun was the primary weapon of Victor Fries which could instantly lower the temperature of its target, freezing it solid. His proficiency 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.
History of usage
After an accident with cryogenic freezing tanks that rendered him unable to survive outside of a sub-zero environment, the man who had become known as Mister Freeze used his knowledge of cryogenics to create both a vacuum tight suit, enabling him to keep his body temperature at 50 degrees below zero (and tripling his strength) and a freeze-inducing pistol, which he called a "cold gun." When Freeze sought revenge on his former employer Ferris Boyle for causing his cryogenic accident and seemingly dooming his wife, Nora Fries. 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, and intended 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 his helmet, and compromised his suit.
When entrepreneur/businessman Grant Walker broke Mr. Freeze out of prison to gain immortality through Freeze's own condition, Walker provided the former scientist with a new environmental suit and a 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. After he realized that Walker was a madman, Freeze ultimately betrayed him and helped Batman and Robin evacuate and demolish Walker's "Utopian" society.
When Mr. Freeze's condition eventually advanced to the point where his body began to deteriorate and reduced him to a head that was attached to various cybernetic bodies, the despondent and maddened villain concentrated on bringing misery to the people of Gotham City by using his freeze gun to destroy their life's work. Freeze was aided by a trio of henchwomen, who were armed with smaller versions of his own freeze gun - unlike their boss, however, those weapons only appeared capable of generating sharp ice projectiles that could be fired like bullets.
Some years later, Batman had acquired three of Mr. Freeze's freeze guns and put them on display in the Batcave.
During the Thanagarian invasion when several Thanagarians attacked the Justice League in the Batcave, Kragger attempted to use a freeze gun on Superman. Unfortunately for Kragger, the Kryptonian's super-breath blew the weapon's discharge right back at him, and froze the Thanagarian in his tracks.
When Inque invaded the Batcave, Batman II used a freeze gun, froze her solid, and incapacitated her for capture. When Inque later escaped from her prison, Batman attempted to use the freeze gun to capture her again, but Inque managed to destroy the gun before anything could happen. However, Batman eventually stopped Inque by using a special cryo-Batarang that was designed by Bruce Wayne, and was based on the freeze gun technology.
After the forty years of his disappearance, Mr. Freeze used an upgraded version of his containment suit that had the freeze guns built into his suit's gauntlets. It was unclear as to when he built the suit or where he kept it in the intervening years. The only thing that he said regarding that was: "It's something I've kept in cold storage". It was possible that, like so many other of Gotham's exotic super criminals, he had one or more auxiliary bases of operations that were equipped to serve his particular needs. Freeze used the suit to attack Derek Powers and Stephanie Lake after they betrayed him, but was defeated by Batman II and Blight.