I have a translated bit of info from German and I believe it's a calculation for progressive rate of twist. Can anyone verify??
? Not without your posting the original German, no.
The twist length was 12 feet (3.766 m) at a twist angle of 3 ° 45 '.
One turn in 12 feet. Thats heavy artillery 'gun', not 'howitzer' barrel (land-based) or "Naval rifle", (seaborne) length, if even the barrel is long enough to accomplish one full turn. Either way, a rate that is probably rather ancient as such things go.
Progressing from 'X' to one turn in 9 INCHES a number sometimes found in small-arms - even handguns.
More info than net twist per total length is needed for establishing 'gain twist' though.
Your degrees of departure would be progressively calculated from their moving base-line, not from a fixed reference, hence the term 'progressive'.
Heavy artillery has taken advantage of progressive twist to reduce stripping effect on driving bands when accelerating the angular velocity from rest of some astonishingly heavy projectiles.
Old idea, BTW:
Report of the Chief of Ordnance - United States. Army. Ordnance Department - Google Books
Others just use a lower rate of twist, 'coz the heavier the projo, the less spin it actually needs.
And then there is the Magnus effect, anyway, so some modern and highly accurate weaponry doesn't spin the projectile
at all. "Fin stabilized" rather than "spin stabilized" and sometimes even "neither one".
Good idea to avoid being on the receiving end of 'any of the above', 'coz all work well enough, and the entity as saved the production cost of gain-twist probably spent it on more deadly shells. Or just more shells, period.