I expect blow-by & patch disintegration is your main problem. For such an undersized ball, your patches are way too thin.
If you are going to use such a small ball, your patches must be MUCH thicker. Guys up here, find satisfaction with a .020" to
.022" patch, either denim, pocket drill, canvas or linen and a ball that is only .005" smaller than the bore, at the most, .010"
smaller than the bore.
Your measurements do not look right for a GM barrel. The bore should be .540" and the grooves should have .012" deep rifling.
That would produce a groove dia. (groove to groove) of .564". Getting a good accurate measurement of the bore is not easy with
calipers.
The best way, if necessary, is to un-breach the barrel, run a rod up to almost the muzzle from the breech end and drive an oversized ball
into the muzzle, then push it out, catch it on a folded blanket and measure the lands and grooves on that ball. That will give an accurate
measurement.
Suffice to say though, that the bore will be VERY close to .540" and the rifling will be .010" to .012" deep for each groove. We've been shooting
these barrels for a lot of years and we automatically go with a .005" under calibre sized ball and 10oz. (.022") denim, or canvas, pocket drill or canvas patch.
This formula works in EVERY gun we've tested. All that remains is to find the powder charge the gun wants. We already know the ball and patch
combination will work with any powder charge you are likely to use. The greater the charge, the higher the pressure and the tighter the combo must be.
That, is a given.