vacuum such as on the moon."

In the paper, they explore the feasibility of using conventional firearms in space (on the moon, specifically).  Some of the items they discuss:

• Guns would probably be able to fire as-is on the moon- gunpowder contains both the propellant and the oxidizer (similar to a rocket engine). In fact, without any air in the barrel, the bullet would meet less resistance, so it would accelerate and move faster
• Recoil could be a bigger problem with less force holding the shooter still. Aligning the firearm with your eyes would also apply torq to the shooter because the force is above the center of mass, so this would tend to rotate the shooter backwards.  They would need to compensate for that. 
• They would need to keep the muzzle velocity below 5500 ft/s on the moon to prevent the bullet from achieving escape velocity and going into orbit around the moon, cluttering it with a bunch of projectiles. 
• Cooling the gun would be a problem, There would be no air to transfer heat away from the gun, and convection relies on gravity so even if there was air it wouldn't cool as effectively
• Also, on the moon each day is 2 earth-weeks long (same with the nights), and there are extreme temperature differences between day and night that could alter the chemistry of the propellants
• In a vacuum, metals can spontaneously weld together, so material selection in the manufacture of the gun would be very important
• In a vacuum, lubricants would evaporate, so it would be hard to keep the gun lubricated

The appendix has a bunch of drawings of possible guns to use on the moon, most of which have very large and prominent triggers that could easily be actuated while wearing a bulky space suit.