Sorry to hear about your difficulties. But enjoy learning thru your reporting of the problem. I was told over 50 years ago to wash my patch material to remove the sizing by a hard core muzzle loader. I had no recollection of every washing my patch material. I guess I should try that. Not that I'm a big ML shooter. I've got the one LH Hawken that was given to me that I shoot when we have an ML match on a holiday or 5th Sunday. So, that amounts to maybe 2, max 3 matches a year.
It will be fun to see how you work thru this, Ian. I am happy to see my Isaac Haines is shooting well for you.
I may have it sorted. Only took a little less than half a pound of powder this time and one flint.
Turns out I was mistaken, my .018" SMR patching has been washed and ironed, but I wasn't using that before, I was using some spare scraps from a resale store that measured .018". Redoing the arithmetic, .3825" groove and .362" bore with a .350" round ball and .018" compressed patch gets me around 3-4 thousandths further compression on the patch in the grooves. Ought to seal. Does seal in anything else I've ever used it in. So I headed out with my smaller cow horn full of 3F, the new .018" patching I use regularly in other rifles, and re-did the usual tests from 25-35 grains using .350" and .360" balls. Dismal groups, some barely on 12x12 target paper. Best in the 5" range. Lots of stringing, but every which direction. Only found a few patches on the ground and there are close to 300 of them out there from this rifle at this point. I stopped for lunch and to rest as the sick fatigue set in and thought over every variable that I had NOT changed since the start. I came up with sprue up, firm seating, using the same short/long starter and rammer end, using water-soluble patch lubes, and precut, square patches. One thing that kept coming back to me as I tried to see where the patches were going was just ONE that I actually witnessed leave the muzzle and it hooked hard to the right at about 15 feet and veered into the ether, more on that in a sec. To recap, I've used three different ball sizes, two different alloys, three different lubes, two different powders (willow and toilet paper), three different patch thicknesses, and the only thing that really changes is the point of impact of the pattern.
Then I remembered my friend suggesting to rip a strip, spit patch, and cut at the muzzle. So off with my patch knife and a couple fresh strips of .018" twill ticking, wiped out the bore, hammered in a .360" ball, cut the patch, rammed like normal, and laid down a 2" group. I also saw my patches going downrange, straight, about 15-18 yards. I'll be dipped. There is an axiom a generally accepted by muzzleload-ors that patch size and shape makes no difference to accuracy. This is backed up by real scientific tests by at least one actual rocket scientist and several others more than qualified to perform valid experiments and make assertions based on the results. Anecdotally, with exactly two years of flintlock shooting under my belt, this has proven out 100% for me as well. Well, THIS rifle is the monkey wrench. It may have just earned a nickname.
For shits and giggles I switched back to .350" balls and 25 grains of powder, got a 4" round pattern. Tried 30 grains, a little tighter. 35, three in a cluster and two flyers. 40, a vertical string (ah---HA!) and 45 a 1.5" cluster. Went to shoot another 45 grain group on a clean target and only had half a charge of powder left in the horn. Sighhhh... It was getting too dim to see well anyway. My cut patches looked good, even at 45 grains of powder. Somewhere in the workup I quit short-starting and just pressed the sprue flush with a 2x4 block, it doesn't take much to start these .350" balls but a bit more than my bare thumbs can handle more than once or twice. Seating flush in instead of short starting made the patch even smaller when cut at the muzzle. I'm not going to speculate about why this rifle won't shoot a 3/4" square, precut patch but will shoot patches cut at the muzzle, and I don't care, I'm just glad it shoots because I was just about ready to abandon the project.
If the weather and my health cooperates tomorrow I'll try to run some more groups and try the pre-cut patches at the higher powder charges just to see if it makes any difference.