20ga Ultra Slug accuracy

[Lost Okie]

    Ok, last year accuracy was 2 inch groups at 100 yrds.  This year, I hunted with my Muzzleloader.  Best group I could get was 6 inches and it was not consistent.  Actions taken.  1.  Replaced cheap scope with Nikon shotgun scope.  2.  Tried different lots of Rem corloks since they gave the best performance last year.  3.  Cleaned barrel....scrubbed with bronze brush and bore cleaner.  3.  Tried Winchester, Hornady and lightfields.  Best group came from Hornady, but still over 4 inches.  Forearm has not been changed last year.  It has an O ring and the first inch glass bedded.  Checked screws in mounts, all tight, watched trigger pull, lockup, kept center to back of forearm on shooting rest.  H E L P....what am I missing

[Markus]

Maybe the crown? Bad scope? that gun should shoot better than that.

[Lost Okie]

Just checked the crown.  No signs of anything, ran fingernail around inside of crown and only felt rifling.  Scope; its new but that does not mean anything.  Although, the groups are similar to the scope I took off.  It was a Tasco 3x9 and would change POI if I adjusted magnification so I always kept it on 4.  Suggestions on how to verify that its the scope ?

[GatCat]

To verify or test scope, replace it with another one that has been good to you.