Is your gun shooting to point of aim? From what you have posted I think this may be the problem. Get a large piece of paper, 4 ft. x 4ft. (newspapers tape to gather and make several) and put a bullseye in the center of the sheet. Place the target at 40 yds. and shoot from a rest making sure not to rest the barrel on anything, only the forearm. Shoot each target (atleast 3) and you can use just small game shells for this. You may find that your pattern is more then a foot or more off target( alot of guns, maybe most don't shoot to point of aim) and you may actually have a good pattern. You can then add some sights to your gun and sight it in just like a rifle.

Geaux get'em !!