BrickArms is a small toy company specializing in custom designed weapons for miniature block figures. I have been using bits and pieces from their Mega Weapons Pack for some of my recent customs, such as the Starjammers:
The weapons are designed for lego figures, but can be easily modified for minimates. To do this, I find the spot where the gun’s grip meets the barrel, and I cut into the plastic about 1/16” horizontally. Then I start at the bottom of the grip and shave off an even layer of plastic until I hit this horizontal cut, and the piece falls off neatly. I do the same thing on the other side of the gun. Sometimes it takes a few tries to get it the right size for a minimate hand. If you are sticking with the weapon’s original color, I recommend painting the modified grip a flat black coat followed up by a dull clear coat (which is actually more of a semi-gloss, which matches the weapon’s finish).
I have tried sculpting my own guns in the past, and I can’t make them look nearly as good as these ones. Their weapons are reasonably priced, especially if you buy the packs. I plan on making a Punisher Armory at some point with all of these weapons mounted on the wall. The upcoming Deadpool minimate could have a lot of fun with these too, even though he is coming with a ton of weapons already. Plus you won’t want all of your AIM agents to have the same gun. Check out all the goodies at BrickArms.com. If you visit the forums you can see a preview of their swords, which I am really looking forward to.