Assuming both barrels shoot the same POI and it is not just one,it is common for double to shoot low for some as they have a tendancy to dip as the shot goes off. In addition,every shotgun ever built will shoot at a different POI for every different shooter. The butt of a shotgun stock is the rear sight.Shooting low indicates the drop at the comb and heel are too great for a particular shooter. ( Mostly the heel) I would guess the heel needs to come up about about 5/16" for the shooter that said it shot 6" low. Might be more or less for you
It is easily fixed if you don't mind the cosmetics.Cabelas sell a slip on cheek piece that as shims in it so a shooter can add or subtract stock height.I think they are about $20.You can also start adding pads of moleskin until you get it right. Typically you want a 60% shot pattern above center and 40% below, but that is a generalization and every shooter wants something to fit his/her particular needs.
