i recently started painting a couple shotguns, cause there easy. good practice. the one in the picture below is simply a test and for looks. If i wanted to acually use it for hunting, i would have gathered samples of near by grass, tree branches and leaves and went from there to the paint store.
Digital is the only real hard camo to do IMO. If your doing digital camo i reccomend buying vinyl stencils off ebay or whatever because using an exacto knife to cut that much digital is insane, i know from experience, it wasn't fun.
the easiest is tiger stripes and its looks great. than woodland, and varients using leaves and branches for the acual stencil. all which can be found on youtube for reference! even watch the kids painting airsoft, there some talented kids out there who taught me some stuff!
good luck.