Well folks, I just got back from shooting again. I'll probably be branded as a liar or lunatic from now on, but can only speak the truth as to what just happened.
Before leaving, I loosened the barrel nut on the magazine tube and retightened it to make sure it was ok. Nothing abnormal. Then, I took off the stabilizer clamp. Everything was fine. Tightened it back up, only SNUG this time instead of torquing it.
Drove up to the woods and shot while seated, @ 50yds as normal, using a wood bench w/ a 5gal bucket on top of it for a rest. Put my left hand under the gun, making sure not to let any part of the weapon touch the bucket, as you all have described. Shot from the same 15rnd box of slugs.
After my 1st shot, I was overjoyed but in disbelief. It shot exactly to POA elevation wise, and was only about 2" off windage, to the left. I mean elevation was DEAD ON. Fired 2 more shots, and got a nice triangle. Measured the group at the widest point and was about 1 3/4". For open sights, smoothbore, and 40 cent slugs, this was more than sufficent for me. Adjusted the windage 1 tick mark, and proceded to put 2 slugs in the same hole. Not a figure 8, I mean in the same hole. Wow. Only problem was that I was now about 2" to the right of my POA. Moved the sight just barely, only half a tick mark, and it shot to POA as good as any rifle I've shot.
The only conclusion that I can draw is that the stabilizer clamp was way too tight and moved the barrel. That may not be true, but it's the only reasonable thing that comes to my mind. Now I can see why competition shooters consider a free floating stock/tube or bedded stock a must, especially when shooting with a sling.
Well anyway, that's my wild story for today, you can either believe it or not. Heck, I don't even think that I believe it myself.