Most patch cutting happens at the muzzles, especially when trying to use heavier cloth to withstand the higher pressures and velocities.
: Denim maintained integrity right to 1,800 fps in my .69. That took 330gr.2F, BTW. At 200gr. and 1,700fps, the same combination I used for 165gr. still worked - ball .006" under bore size, and a .020" patch. My rifling twist was 66", and yet didn't strip or burn patches.
: Measure your groove dia. Measure your ball dia. Subtract the ball size from the bore size and divide by 2. That is the thinnest patch thickness that might work with pure lead balls and 3 dram (82gr.)loads. Increasing patch thickness to about a .005" compression per side (or more) may be necessary for work for heavier charges. My rifle did just fine with .005 compression per side, .010" total.
: The heavier the charge, the higher the pressure and the tighter must be the patch. Polishing the crown into a smoother radius or slight cone shape at the muzzles will help you to load a tight enough combination.
: Slugs must be .001" to .002" larger than the bore size so they centre and are held by the rifling. Undersize bulets are strickly a NO/NO. Even at that, firing one barrel will likely move the other forward, which is a VERY dangerous situation. Please use only round balls due to this problem.