The twist is about the bullet weight. Not the propellant
Bullet Weight --> ~ length
Black
Powder --> ~ speed
Miller's is all about length/diameter and speed re twist.
I one looks at "The Internet" one finds all sorts of opinions as to 38-55 heavy weight reqm'ts and performance... ranging from 12:1, 14:1, 18:1 ... and all sorts of "that's too slow a twist" for BP speeds. In fact, straight Greenhill's likes a
14 twist for those lengths/diameters/speeds Still, things are a bit more complex that straight Greenhill.
So like UncleNick, I went to both JBM & Berger w/ real data on real bullet and real lower-bound speeds and plugged it in for an 18 twist:
. . . and it appears that theory and actual shooting data do overlap.
.