No Prime needed in most cases. Just sanding if it's Krylon.
There's some that you may need to rough up a bit more than normal, but a fine grit sanding should get it done enough for the paint to stick.
If you're an automotive painter, you could pick up some DuraCoat professional level gun paint (
http://www.lauerweaponry.com ), use an HLVP and spray your guns with whatever colour you want from them. Or use a camo pattern kit. Then bake it in your shop (if'n you got that kind of access). It'll be super primo.