post: I lied.
QL predicted 1,155fps from an earlier/lower trial from which I'd tweaked the Miller K-factor for Swiss 1.5
LABRADAR showed actual to be 1,159fps
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.
My .38-55 has a 15 twist by rebore from .32-40.
It does well with the 336 gr Lyman "Mini-Snover."
I load 40.6 gr Swiss 1 1/2 because that gives compression the thickness of the over powder card wad with the top lube groove just covered.
Won't knock down all the 500 metre rams I hit, though.