Shoot the gun in a vice, lead sled, sand bag, etc. It could be that you are pulling the gun when shooting. However, shooting all those times it is very unlikely that you are. I know that I had patterend the magnum blend in my SBEII twice and pulled it right both times. The last time I patterend it with sand bags and it pulverized the paper. I took mudhen's advice and set up at 40 yards with a 10" circle and a 30" circle outside of that. I had 577 in a 30" circle at 40 yards. The gun did shoot high, but I am going to get some fiber optic sights to put on that will lower it some.
One question is your choke ported like the picture I posted, with the port holes?