If you are consitantly shooting high, definitely use the shims. A good way to test if you are on once you put one in, is hold your gun like you would in your blind at the ready but not shouldered. Look at the corner of the room up at the cieling. Shoulder your gun pointing like the corner of the room is the bird, but do not look at the gun, only at the corner while you mount your gun. Hold it steady and then check your bead. Is it on target, and do you see any rib on your gun. If you are shooting on target, when you pull up you should see none of your rib at all. Only the bead and the target. If you can see the front half of your rib(even just a fraction of it), which I suspect you would right now as your gun stands, then you need to use the shims to drop the front end of your gun down. Like I said, it should be 2nd nature to just keep your eye on the bird, dont even worry about the bead. If you have been shooting a 870 for that long, then you know shotgunning, its just the new gun isnt adjusted to your normal shooting style. Everyone's shooting style is different, and its very hard to change. Much easier to change the drop on your new gun
does having multiple screen names on a forum and having a conversation with yourself make you an extra special rock star hunter?