I very much like Minwax clear satin water based Polycrylic. I cut with 25% water and apply very thin coats using a 8" square of clean cotton cloth balled up into a puff ball. It dries almost instantly and can be sanded within fifteen minutes. I apply at least six coats, wet sanding very lightly with 600 grit paper between coats. It gives you a durable finish that's also easy to touch up if the gun gets banged around. Be sure to put a coat or two in the barrel channel to seal it.

http://www.minwax.com/wood-products/...tective-finish