I thought this was called "The Muzzleloading Forum "? A place to discuss "MUZZLELOADING". Yes, I am a purist. I only shoot muzzleloaders. I didn't join this forum to discuss cartridge guns be they black powder or not.
If the rules stated muzzleloaders only, you were in the wrong regardless if your gun was period correct. Would it have been okay if guys showed up with Henry's, Burnsides, or other CW cartridge gun? I think not.
Long bearing surface bullets require fast rifling to stabilize the bullet in flight. 500 plus grain bullets sometimes won't stabilizing until they're out 200 - 300 yards.
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If your patches are shredding it can be one of two things. The rifiling is new and sharp and is cutting the patch allowing blow by or your patch is too thin. I believe you are dealing with the latter. Cut the charge back to 50 grains and try a thicker patch. I think you'll see improvement.