Formulas usually only work for on small range of bore sizes. 1/2 ball weight works great for some calibers like 44-50. And its documented to the Revolutionary War period by the writings of Col. Hanger. But is too much for some calibers and not enough for others.
A 32 caliber may need 30-35 to shoot well, the old one grain per caliber formula will be ball park in this bore size. In a 69 caliber is wrong with both the ball weight and the caliber formulas. My 67 caliber shoots a 437 gr ball at 1600 fps with 140 gr of FFg. This formula (about 1/3 ball weight) with a ball weighing 50 gr would give very poor performance though I have used 15 grains in a 32 to shoot rabbits setting under bushes in the grove when I was a kid. Very low velocity but head shooting rabbits at 10-15 ft does not need much velocity. In a 40 cal the the one caliber per grain is a little light for some rifles, though many may shoot well with it. Up to about 54 we are back to the 1/2 ball weight again. But by 58 it starts to be a little much.
Twist is usually irrelevant unless VERY slow even by RB standards. A 72 twist MAY shoot better with a little more powder than a 48" twist 50 cal but either will accurately shoot all the powder necessary for 1900 fps velocity if this is decided. I would prefer to have a 48" twist to at least 54 caliber but moderns have lost sight of this original twist rate and most modern barrels are slower than necessary.
Dan
