If you want it rough and ready just mask off critical parts and go for it.
If you want lasting easy finish that covers everything degrease it with brake cleaner.
For a sharp finish you'll want to take the gun apart, key parts with scotch brite, scrub all parts in fairy dish solution, then spray.
Whatever you do use multiple light coats, 15mins between coats so it dries and adheres properly.
In order of quality;
Humbrol aerosol
Krylon
Kings army
Halfords camo