Could one take a tank of liquid nitrogen (or similiarly cold substance, say liquid hydrogen ;)) and a gun that fired a centered spray (that is, maybe a diameter 10 cm at 15 feet) and use it as a close quarters (obviously lethal) weapon? What would make it not work? If it did, what would some countermeasures against it be?
The liquid would quickly vaporize. Not only would it obscure your vision completely, it will displace the oxygen and suffocate everyone in the room. If you are standing outside the room, it might work as a weapon. Still, not a very effective or practical weapon. A simple grenade would probably do much more damage, and doesn’t need to be kept in a freezer.
I can think of lots of reasons why this wouldn’t be a great weapon.
a) Ugly. It would cause really ugly wounds if it wasn’t lethal. Frostbite, gangrene, the works. So it would probably be outlawed by the Geneva Convention.
b) Totally imprecise and prone to splatter. You’d stand a good chance of getting a face full of it yourself.
c) Heavy and bulky. Which would you rather carry, a .38 or a 38 pound tank and gear?
d) Really bad in a firefight. If you caught a projectile in the tank, you and everyone around you would be toast. Or actually, the opposite of toast.
The weapon doesn’t sound like a good idea,for the reasons already stated.
However,police forces and armies do try innovative new weapons often to accomplish specific kinds of tactical objectives.One that I can think of is a gun that sprays sticky foam at a target.(IIRC the foam is a polyurethane compound)This type of weapon is used to subdue an opponent and take him prisoner.Think of flypaper for full grown people to get an idea of how it works.The targeted person is quickly rendered immobile.
Another type of innovative approach is to use bullets that disintegrate when striking their target.This too is a nonlethal weapon which I have seen used on television to shoot the gun right out of the hand of a beligerent drunken suspect with a handgun.When his pistol was struck with the “magic” bullet it flew from his grasp,and officers rushed him and quickly subdued him.
The main point here is that these types of weapons are used to disable an opponent and facilitate taking him/her prisoner.
Perhaps someone with a lot more knowledge than me will come along,and enlighten us all concerning new lethal weapon ideas,but these are the two that occured to me when I read your OP.
German riot police used cold as a weapon in the 70s, sort of. They used watercanon on rioting crowds with a couple of chemicals mixed into the water that caused an endothermic reaction. This made the water icy cold, and really knocked the fight out of the crowd. It’s tough to be a menace when you’re fighting off hypothermia.