Nice to hear the Gun shoots well! Normally the Gun has to be adjusted to 100 meters (about 109 yards). The ejector of the shotgun may need some inspection or adjustment. Or you have to change ammunition, is it plastic or paper shells?
Why don't you shoot the slugs over the scope? Didn't work? With slugs you always have the problem finding the right ones! You have to test different makers, but I set on the Brenneke, because they surely are the best.

The Crown over U means "Untersuchung" = "Proof approved"

Best Regards,
Gunwolf

http://www.hunting-heritage.com