I want to stay away from inline and have decided on a caplock.
First off, welcome to the forum and I like the way you think. .....
You are asking some very good questions and in the right place. Keeping in mind, that mass kills, especially when working with RB's, you will get closer to the performance you want, with the .54. Load it with a Honady Plains bullet or Max-Hunter and you have a killing combination.
These are classified as Conicals, using no patch or Sabots. Just a bit of lubricant.
It seems most 50's i see are 1:48
Sadly, this is true and done so, to "somewhat" cover a range of projectiles. I look at some M/L's and they speak, 1:60 or 66. Definitely RB shooters and then read 1:48. .....
Twist as about stability where a Conical needs faster twist to stabilize and a RB, not so much. Just think about one should throw a football .... "Spin"
Be Safe !!!