The Lyman GPR is by far the best RB rifle I have ever owned. The 1 in 66 twist with a .010" riflings really hits the spot. At 70 -75 grains FFG off a bench you can shoot 2" groups all day if you swab the barrel often.

The twist rate on any rifle is to spin the bullet and stabilize it during flight. The length of the bullet and the velocity of the bullet are the deciding factors for twist. If you shoot a bullet that is 2-3 times the bore diameter in length, then you need a faster rate of spin 1 in 28 - 1 in 36. You can compensate for an improper twist rate to bullet length by varying the velocity. That is why every rifle/load combination has a sweet spot.

The rifling depth is to imprint the cloth patch. seal the bore and make sure you get all the spin your supposed to out of your bore.

If you have to err on twist rate it is better to err fast. A 1-48 twist rate will do an exceptable job with a RB. as long as you don't try to push it too fast. Try 70 - 75 grains with your 50. This load will shoot completely through a mule deer at 75 yards. I know.

To stabalize a round ball or any projectile for that matter you need to get enough revolutions per second. To do this you can do a faster twist with a slower speed or a slower twist with a faster speed. If you are going to be shooting at distances of 100 yards and beyond I recomend you go with a slow twist with faster speed for flatter trajectory. If you are going to be shooting a maxi-ball type projectile a 1-48 will work fine at 100 yards with the correct powder charge.