First of all, I have always used Bushnell scopes, and currently I have a trophy shotgun scope on both my 1100 and my 870. I have never had an issue with either of the scopes. Granted, I think that if you go below the trophy line of scopes you've gone to cheap, and you might have problems.
Both my shotguns have rifled barrels, and I tried many brands of slugs before I found one that works for my guns. Actually there are two slugs that work, but the first, the Remington brand Buckhammer, although accurate (2" @ 100) is very abusive on the shoulder, not to mention the shotgun. The second slug and the one that I used to shoot a buck at 150 paces with is the Barnes tipped expander, loaded by Federal, premium brand. With the Federal slugs I get 1 1/2'' to 2" consistently at 100 yard. Full bored rifled slugs work in a rifled barrel and shoot fairly accurately for the first two or three shots then the rifling gets so fouled with lead that you might as well be shooting a smooth bore. I don't reccomend you try these slugs because trying to get the lead out of the rifling is near to impossible.
Going back to the buck I shot, not only are they very accurate, but very powerful as well. The velocity on the box is 1900 fps for a 3/4 ounce slug. At the measured 150 long paces the slug hit the deer in the shoulder piercing the lungs but not the heart, but still the deer went down right away, then got up and ran ten feet, collapsed and was stone dead by the time I got there a minute later. Alse the slug broke both shoulders and exited stage left. Too bad I would have liked to have seen if it was a text book expansion. From the exit wound one could tell that the slug had expanded, but all the same the damage was not that extensive and the meat around the hole was not a write off. I love these slugs.
