Well, I spent the better part of the day shooting with the scope but had some trouble getting decent groups. I had a few other tins of different pellets this time so I took the time to try them all out. Again, the looser ones grouped much better. The Daisy's came first, then JSB Exacts followed by Gamo Match, which surprised me since the quality of the Daisy pellets aren't that great. The Gamo hunters grouped much better than CPHP, but they too fit a bit snug unlike the loose fitting match pellets. With the scope on and shooting at 10 yards however, my groups weren't really improving at all. I would get a decent group going for the first few rounds, then there would be sudden shift (huge shift), which would group good another few rounds (rested). I had it down to 2 possible things; either it's my hold/position changing or the scope/mounts are shifting. This also makes it hard to find the sweet spot and I've been trying every inch from the trigger guard forward, but she continues to make a small spot of decent groups, then shift.
I made it a point to pay attention to my shooting position and all the little details to try to pinpoint the problem, as well as to keep an eye on the marks I made on the mounts prior to shooting, but the problem persisted and it didn't seem anything was shifting. I decided to remove the scope and mounts to see if any damage had occurred to the soft scope rail. There was none so I cleaned her up really well and decided to re-mount the scope using 2-piece rings instead of the 1-piece it was on. I added a scope stop behind the front ring for added support. Prior to mounting the 2-piece rings, I shot about 50 rounds using the open sights again, and my groupings were much tighter than with the scope on. I put a small soda bottle filled with water out at 25 yards and fired 5 rounds at it from the kneeling position with open sights. They were all a hit and went clear through (and that was using the pellet it shot worst with - CPHP).
Another possibility came to mind after this and it's that the weight of the scope (it's pretty friggin' heavy) may have changed the recoil pattern of the rifle to where it's making it really hard for me to find a good shooting position. Well, the new mounts pushed the scope about 1 inch forward and are a little lighter thant he 1-piece. Today I'll put another few hundred in to see if anything improves. The scope is fine, but there may be some shifting and settling of the mounts/scope I'm not aware of that's affecting my groups right now, or possibly just the weight of the scope slightly affecting things.