It's called hydrographics.
The gun is prepped with a base coat of paint (to give the base color) first.
They print a pattern onto some sort of water soluable film, lay the film in water or some other solvent, then dip the firearm into the liquid.
The ink transfers perfectly onto the firearm.
I had a shotgun in Realtree Hardwoods and whatever base coat they use is pretty darn durable. My guess is it's some sort of catalyzed epoxy or polyurethane finish.
The stuff looks good, it's more durable than your typical paints, and is water tights so you dont have to worry about rust on the outside of the gun.