I guess if the gun barrel wall is rated to survive a peak internal pressure of X (atmospheres) inside it (on earth) then when its fired in a vacuum the pressure on the barrel wall goes up to X+1. Small bits of material in orbit can cause alot of damage so the remains of the propellent coming out the barrel would constitute a hazard for other things in orbit (which the projectile didn't hit). I reckon a rail gun is the better bet. Dunno about the gun+spacesuit pic. I'm thinking you might be right about it being photoshopped!

The atmospheric pressure pint is irrelevan All gus are tested at excessive pressures way beyond 1 atmosphere. What would be a problem, however, would be firing hand held guns. The recoil would be likely to send the firer head over heels backwards.

Overheating may be a problem. There's no air to take the heat away, and simply radiating it is slower. SC-

Spent cases could also float away into awkward orifices on the spacecraft.

A high power air rifle would not have the case or recoil issues.

you could have a tiny little recoiless piece, or perhaps something like the gyrojet from the 60s main problem i could see is lubricants. most of the gun oils etc would freeze/ sublime away.. youd have to use graphite. not sure how good that would be an airgun still has the recoil problem, but i suppose you could fit a reaction jet to the weapon, so when it fired it would dump enough air backwards to negate recoil entirely

er, why not the recoil issue? you could vent some of the gas aft like a recoiless rifle but thats a little inefficent. Spent case, sorry - it was in poor taste.

why didn't I think of that...sooner!

That's all right. I did a double-take on that, but it's all in context. SC-

"er, why not the recoil issue?" Depending on how the air gun works, if it has a piston to drive gas down the barrel (such as mine), the piston moves in the opposite direction to the shot, and so the two balance each other out motion wise. So, a space airgun could consist of a barrel, with top or side mounted airtight magazine, a piston tube bellow the barrel, and an airtank below the tube - which would let some gas into the piston tube on shootiing. Could even use the gas tank for breathing oxygen if you were running low, or even direct the oxyge via a tube from your breathing supplies, but that would surely make people hesitate before shooting :D