Hi,
I'm currently reading "The sporting rifle and its projectiles" written by James Forsyth in 1860-s and he talks at length how people choose smoothbores over rifles (given the same caliber) because manufacturers make rifling with twists too fast to shoot PRB with heavy hunting load accurately at distances around 60-70m. As an example of twist he considers too fast he gives 1:42 in a 13 bore (.71 cal) rifle. He talks how when loaded with 48 grains (1 and 3/4ths of a dram) of powder it was accurate, but with even a little more it would strip the patch. That sounds to me like an extraordinary claim.
Perhaps the powders they used back then were measured by volume and they were much denser than what we have now. He talks elsewhere about a great rifle with a twist rate of 1:104,. 69 cal (14 bore) which he loaded with 137 grains (5 drams) getting muzzle velocity of 1600ft (calculated from ball drop he gives). With a modern powder I get same speed from a. 58 cal with a ball almost half his weight with 120 grains. So we can probably assume they had powders from a third to double the energy of ours.
Still, even if we take that doubling into account his claim is that using today's powder one should expect to not be able to use more than 90 grains in a. 71cal with a 1:42 twist before PRB would strip the rifling and be "no more accurate than a smoothbore". He also claims tight fitting balls and patches that have to be "hammered in" the barrels don't improve this.
I'm reluctant to dismiss authors experience of many months of shooting in India, and at the same time it sounds wrong.
I would very much like to find out what is the maximum radial acceleration a patched round ball combo can take before slipping. Modern mechanics tells us the tighter the ball-patch interface is the higher the friction and the higher the force it can impart on the ball before slippage.
Personally I haven't observed any deterioration in accuracy that can't be explained by my handling of the recoil with my. 58 cal even with 150 grains. It would be very interesting to find out what other people observed. If you found what your rifles maximum accurate PRB load is please reply with the caliber, twist rate, barrel length, ball size, patch thickness, lube type, load and powder type. Also muzzle velocity if known.
If I have few answers(ideally from people with different caliber rifles) I'll try to find out if one can calculate when this slippage will occur based on caliber, velocity, patch thickness and twist. I think having an equation like this would be very useful to many people.