Bob,,,,
I have a Mauser 93 which is tumbling the boolits and my best "grouping" is something like
12"......I checked the free-bore and it was almost 7/8 of an inch,,,which means that my boolit is having to jump this gap before it hits the rifling. I think in my case this excessive free-bore is what is causing the tumbling and inaccuracy. Just a thought to pass our way.