In Serious Sam, the pistols were equipped with Techno-Magical Ammunition Replenishers which teleported bullets into the chambers each time you ran dry. A Replenishing mod, which magically allows the gun to be reloaded with normal bullets as a standard action (maybe move-action if it's a one-shot pistol) would probably be worth a +1.
In Preacher, the Saint of Killers had guns that would never miss their mark, nor would their hammers ever fall upon empty chambers, nor would they ever inflict a wound that was other than fatal. That's probably a bit out of the scope of regular D&D gameplay, though.
Perhaps an enchantment could allow using Manyshot with the firearm, a la Unreal Tournament's rocket launcher. The value would probably depend on the firearm, tho... for an automatic, +1. For a pump-action shotgun, +2 or +3 most likely.
Perfect Dark had teleporting-bullet rifles that could jump through walls to hit their targets, but I guess that would be covered by Brilliant Energy. The guns also had x-ray scopes or something (otherwise, not a lot of point in teleporting bullets!) but I would call a scope a separate item.
Don't forget the ever-traditional silver bullets for killing lycanthropes. Rounds of darkness or silence could be pretty useful, too. I'll bet I'm forgetting a ton of stuff, but hopefully this will help!
