Here is a .58 flint fullstock Hawken I built. The flint chamber was about 3/8" in diameter and powder bridged at its mouth. With Swiss 1 1/2F it flashed in the pan about 35 of 50 hammer falls. I drilled the chamber out to 7/16" and radiused the mouth and polished the chamber. It then was reliable. This smoked dowel shows the size of the powder chamber before enlarging (I think).
I drilled and tapped the .58 cleaning jag face to take a .40 caliber cleaning jag. With a long patch on it (like a band-aid), that cleaned the chamber. The patch was also held on the .58 jag so it didn't stick in the powder chamber.
To drill the jag, I chucked a bit in my drill, pushed it down into the vice jaws, clamped it, released the chuck, clamped a jag in my chuck and drilled and then tapped for the smaller jag. Worked fine.
If the patch did come off, this ball puller got it out.