Here's the gun
I went out shooting yesterday with friend (Dead eye Lefty) who was kind enough to give me an introduction and some coaching for cowboy action shooting and fast draw. He had some 125 grain Tightgroup loads, and I fired a string in my rifle. They seemed to shoot pretty good. My 158 RNFP Trailboss loads in his 66 shot poorly...
Low and behold when I got home some 125 truncated cone Rimrock bullets I had ordered showed up. I decided to load a few with the same Trail Boss load in the same brass (357 Remington) as a comparison.
Massively more accurate than the flat point round nose. I was really shocked. The one flier on the right target I believe was me. Just shooting off a field rest.
Anyhoo, I don't think I'll be needing to return the rifle to Winchester, and I went ahead and ordered the sight, screws, blank, and a spring for my Stoeger coach gun. It will be interesting to fire the rifle from a proper bench with that load. I guess my gun just doesn't like 158 RNFP or 158 SWC. At all.
Thanks again for all the great info and replies.