I've experimented with heavier and lighter charges, with 3f, 2f, pyrodex RS, and 777. I've used .490 and .495 balls, with patches ranging from REALLY tight (.018 pillow ticking + .495s) to Granny's old cotton nighty (<.010). I've used Lehigh lube (best so far), bore butter, Hornady's version of bore butter, SPG, and half a dozen home brews.
Virtually all the patches I recover are badly damaged. The patches are not damaged during the loading process, only after firing. I'm sure the patch damage is the red flag for this problem, leading me to suspect a poorly finished barrel. I hoped it would "break in", but after 600+ rounds, I still have the same issues. I'm leaning toward lapping the barrel, but haven't found the time.
Frosty- what's the best way to measure a barrel to determine if it's centered? If it isn't, what change would it make in accuracy?
Arcticap- I'd heard they liked heavy charges, too, but not in my experience so far.
BTW- several of my firends have GPRs of older vintage (12+ years) that shoot wonderfully- even when I'm behind the trigger. Mine is an '04. This makes me wonder even more about the quality of the barrel finish.
Thanks for your ideas.