RE: Slug gun help needed!!!
I don't believe you should put the nut on that tight, it should actually say in the manual. If you don't have one check online for one you can read. Being that it is a cantilever mount barrel the fact that the barrel may move a little probably doesn't mean much since the scope is fixed to the barrel, not the receiver. If the scope were mounted to the receiver then it would make a huge difference. Which is why I don't like receiver mounted scopes on shotguns unless they have pinned or fixed barrels.
The problems you are seeing could be numerous. One thing I see right off the bat is you are not shooting a group before adjusting the scope. It sounds like you are shooting one shot, then adjusting and firing another. You should fire at least two or three shots before you adjust anything. I know that gets expensive with modern sabots though.
And like mentioned above most rifled slug barrels have a 1 and 30 something twist to them and don't seem to like the faster rounds. Try some ammo that shoots closer to 1700 fps and see what happens. Another thing is slug guns can be very picky about what they like, you just have to try several brands and types to see what works best. And shooting relatively well up close and going to hell in a hand basket at 100 yards or farther is fairly common as well. This is why you should ALWAYS shoot your gun at the ranges you intend to hunt game at.
I have an H&R ultra that will do the same thing with some loads. Shoots great at 50 yards with just about anything I feed it, and most are pretty darn good at 75 yards. Then some of them won't even stay on a 8x11 sheet of paper at 100 yards! Mine simply does not like the faster Barnes bullets by federal.
The fact that it shot better when it was cold is a bit puzzling and that would lead me to believe you had the nut too tight or something else is going on.
Could be a scope failure as well, I have dealt with that recently and it's enough to give you fits.
If you don't figure it out let me know, I know a site that deals specifically with slug shooting with some very knowledgeable people that may be able to help you out.
Paul