I don't know the solution, but I liked reading the suggestions. Since you changed powder, changed patches, ran scotchbrite down the barrel, I would say the crown issue is a strong possibility. I hope you get is solved quickly. Good luck.
My Green Mtn barrel on my under-hammer match rifle did this (even after a modest fine grit lap job) for a bit but after about 100 shots it stopped and has never reappeared. I've plug gauged all my GM barrels and have never seen such uniformity in bore diameter in any other brand.In the attached photo are some of the most recent patches I have shot with my Kibler Colonial Rifle which has a 50 caliber Green Mountain barrel. Here is a little history and then my question.
These patches where shot using 80 grains of my home-made BP which is in the 2F-3F grain size, patch material is denim .022” measured with digital caliper and cut off at the muzzle, lubed with Ballistol/Water 5 to 1 mixture, balls were Hornady .490. These are the best-looking patches I have shot with this rifle in just over 100 rounds fired.
I started using my cast .490 balls and have used some Hornady .490 and .495 balls and have used several different patches .010”, .015” .018” sizes made of drill, ticking and other 100% cotton material. I have used Wonderlube, Bore Butter, Spit, and Ballistol straight and different water/Ballistol mixtures. All of these ball and patch and lube combinations have given me completely shredded and destroyed patches, so much so the patches were not recognizable as patches but just shred pieces of cloth. I have used Goex 2F as well as my home-made BP. I have scrubbed the barrel a total of about 250 in and out times with Maroon Scotchbrite. I have attached a .490 ball to a ball extraction screw tip and pushed it down the bore with a .015” lubed patch and extracted it to see if there was any damage to the patch by rough rifling and while I can see rifling marks on the patch there is no damage to it.
I have been shooting traditional muzzle loading firearms for over 50 years. One Lyman Great Plains Rifle, flintlock, Two T/C Hawken style rifles (percussion), a Jonathan Browning Mountain Rifle (percussion), Traditions Hawken Woodsman (percussion) and several BP revolvers (all percussion) and various other rifles and pistols (percussion and flintlock) owned by friends. I have never seen patches destroyed like they have been with this rifle. I have seen some undersized patches that have been burned through, but nothing like this.
I am also quite concerned about accuracy, because I can get no consistent grouping like my other rifles. So, my question is - What do I need to do to fix this? I would appreciate any suggestions to remedy this.
Thank you for your comments.Those patches are definitely burned to cinders. Holes in patches can usually be traced back to a sharp crown that is yet to be smoothed. Burned patches indicate the patches aren't thick enough and flame is funneling up the grooves and burning the patches. Filling those grooves with a bit of patch compression is necessary. Once the muzzle crown has been worked to a smooth, rounded entry into the bore heavier patches can be used. You should not have to resort to weeny powder charges or prb buffers just to get decent rifle performance.
Sorry but I don't know what type of rifling Green Mountain uses. Likely someone here will know.I didn’t see what type of rifling you have. Round bottom use a thicker patch to fill them. Even so , i use .015 or .018 in my 58 caliber with round bottom.
I too think you should back off the powder charge.
I too pulled balls and patches and found no holes.From suggestions from others on this forum I have started and pulled 6 .490 balls with 3 different patches and thicknesses, .015, .022, and .035. I have used two different lubes, Bore Butter and Ballistol/water mix, and twice with no lube. None of them have shown any cuts or abrasions on the patches. So I don't believe the crown is cutting the patches.
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