Duracoat is used to paint firearms. It's a 2 pack paint which ideally needs baking so marker should be stripped to protect seals etc.. Another brand that works well is Krylon Fusion which is popular in the airsoft community. You can either mask up to get the camo effects, or if you have access to a small airbrush you could freehand the pattern. This gives a better blending of the different colours, and more realistic effect. It's a good idea to seal it afterwards with a matt clearcoat, as the fill used in paintballs can soften and lift the finish.
what guns are you painting? if they're anno'd its tricky, if not (e.g. Tippmann 98's) it's reletively simple, have a look on UK Scenario in the gallerys.
Duracoat is good but after a while it can peel, ive seen some one trying to sell a black and yellow duracoat'd ego 11 on Pbnation and no-one was buying it cos it was peeling and ruin'd it