Do it yourself jobs turn to Brownells Aluma-hyde.  Best there is in a can.  They have a few good colors: Olive Drab, Coyote tan, Flat Black, ectt.  Even have a black grit spray for forearm and palm areas.  Prep work is the key and they even have stuff for that too.  They have very knowledgeable customer service.
Keys:  Completely strip gun apart
           Proper prep work by removing ALL oil and grease--Contact Brownell's
           Tape up everywhere you dont want the paint.
            Warm conditions to paint (Above 72 degrees minimum, but i think 80 is way better) and              adequate time to cure properly(depending on environmental temp. , but 30 days is good)  
            
Their stuff is as tuff as you can get for do it yourself applications and impervious to most all strong cleaning products and solvents