Firearms skill. Machine guns tend to do large numbers of very small dice (usually 10d2). This gives them the highest minimum damage out of all the firearms, but the lowest maximum damage. Like all guns, a high firearms skill is needed to use a machine gun properly.
Both machine guns and shotguns look the same in appearance. (Elona+ adds a new sprite for Shotguns).
See Ranged Weapons for distance modifiers.