Is your barrel rifled or smoothe? That makes a difference on accuracy. Rifled barrels love sabot slugs and smoothebores love rifled slugs. To be perfectly honest, at 40yds, your groups can and should be tighter than this. I shoot copper solids from a rifled 870 and my 100yd groups look about like your five upper shots.

What gun are you using? I use copper solids out of a single shot H&R and get incredible groups at 100yds. I am assuming you are using a scope? If that is what the coppers are giving you at 40 with a scope, I would keep looking for a better round.

I dunno man, maybe you need to look for different slugs then. Sounds like you got the right set up, but the groups should just be better than that I think. I haven't done it for awhile, but at 40yds I'm pretty sure I can make one ragged hole with 5 shots. Try different slugs and report back.

Sounds like a good set up- if your rings are tight and the scope is in good shape, I would try out some different rounds. If the scope was getting rattled I would expect a bigger variation than what you are getting. You should be getting much better groups at 40 yds. Something is going on- worst thing that can happen is that you get to shoot a bunch more!! Good luck.