Your gun is talking to you. Any blown patches mean that something is wrong.
If the bore is smooth, as you said, and you are getting cutting, it's the crown. Look up member Daryl here and there are numerous posts showing how to polish the muzzle crown by hand. Very easy, and a smart thing to do.
Pre-lubed patches have never worked for me. I have been told that the lube breaks down the cloth over time. Also, I haven't seen any cloth weave I'd trust used in pre-lubed patches.
I use The Minute-Men patching canvas (untreated), and have never had a burn-through or failure. I recover all of my fired patches I can find, and hold them up to sunlight. If I can see any pinprick of light, I look for the problem.
Doing some math, you GM barrel runs .50" bore plus a .012 groove on each side. That's .524 to seal. Your combo of a .495 ball plus .015 patching (all around) is .525. That is barely sealed, and most likely not sealed in the corners. You'll get blow-by, affecting velocity and accuracy, as well as getting a harder bore cleanup after shooting. I run a .495 ball with .018 patching, giving .531, enough to seal. The inconsistency you find may be due to velocity variations. Blown patches do not seal, and guarantee inconsistency. Your test of a .490 with a .015 patch is not sealing. That is .520 in a .524 "hole". The gas jetting by the patch burned and cut it up.....blown patch.
70 grains of 3f Swiss is not out of line, but I think 65 to 70 grains of 2f is more like an accuracy load. Swiss is hot compared to Goex, and 3f is hotter than 2f, so you are moving the ball pretty good and pressuring your patch material.
You heard 2f is "faster lighting" than 3f? I am not sure what that means? In Pletch's test on using various BP grades for priming, NullB (dust basically) was fastest igniting, followed by 4f, then 3f, then 2f. The pan flash is usually what people care about being fast, and I'd use 4f if I were you. The main charge for your gun should be either 2f or 3f. 3f gives more velocity, but may not give more accuracy. I was taught .50 and up best used 2f for accuracy, but there are some who find 3f works well. My thought is, "What good is speed if I can't hit what I am aiming at?"
It sounds a bit like you don't have a solid plan for testing your loads. Start with one combo, like a .495 roundball, a .018 canvas 100% cotton patch, and 50 grains of 2f. That is sort of a light load. Change ONE item at a time so you know cause and effect. Run the powder up 5 grains between each group. Look for where the group gets small, and then starts to increase again. The small group is your charge. Also, if you change ball or patch after finding the charge, do the math first. No sense in hoping a too loose combo will seal when the math tells you the truth. Again, change one thing at a time.
I use Daryl's method of wet lubing, and it works. No sense in wiping between shots (I started that way, but found Daryl's way better). When you wipe between shots, you are simply pushing all of the fouling down to the breech. It then gets baked on by the next shot's load. It builds up, and you have to scrape it from the breech face (you are doing that in cleaning?). When you wet patch, you pour the powder in, and use a wet patch wrapped around the ball, which wipes the bore as it goes down. It puts the fouling on TOP of the powder, where it gets blown out with the ball. You save a step, and don't store up the fouling at the breech. Works like a charm. I use Hoppe's Black Powder Lube (NOT No.9) or Lehigh Valley Lube to wet my patches for anything other than hunting. For hunting pure mink oil seems to suffice.
Once you get the muzzle smoothed out, it'll load a lot easier. You will need to decide the balance point between accuracy and tight loading for yourself. But do the math first. Wishing is fine, but doesn't work in loading longrifles. :-) :-)
I hope all of this helps a bit. Happy to PM offline. God Bless, Marc
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I must preface this by saying I'm not exactly proficient with a flintlock. Its the one rifle that's beaten me over the years.
I've tried a .495 ball with .015 patch ( Ox Yoke pre lubed) with 70 grains of 3f Swiss. She does cut patches, even with a wad over powder. Accuracy is good( ish). But not very consistent. This could be me though. This combo loads very tight. Too tight for a hunting load. Group size ranges from 1 1/2 to 3 inches for 3 at 50 yards.
Tried a 490/.015, again with wad, and same charge. Loading much easier, but still cutting patches. Suspect the crown as the bore is quite smooth. Accuracy was good, but again inconsistent. Group size about the same as the .495/.015.
I tried 70 gr of 2f Swiss with the .490/.015, with wad. Had 2 flyers, one blown patch and clueless about the other. Group size was larger, 1 &15/16 smallest, to 2 13/16 largest for 3 at 50. Ignition with 2f is percussion fast. I had heard 2f is faster lighting than 3 in flintlocks. It is in mine.
I swab between shots with rubbing alcohol.
All groups show 2 shots at or under an inch with one shot spoiling things.
I'm going to try a .490/.018 next. I believe it will strike a balance between accuracy and easier loading.
I'm encouraged so far. This is an accurate rifle and very comfortable to shoot.
We're going to get along just fine, once I find the load.
