Some info on the M91 Cavalry barrel and its twist.
I've seen a few variations on the specs for the gain twist barrels found on the M91 Cavalry rifles we've been discussing, so I went to an Italian site to try to get closer to the "source", and used an Italian to English Translator to glean some info for us:
Barrel Length - 451 mm - 17.76"
Rifled Length - 375 mm - 14.76"
Four grooves Right Hand Twist
Twist at chamber end - 38.96 cm - 15.34" 3 degree inclination
Twist at muzzle end - 19.00 cm - 7.48", 6 degree inclination
I suspect that the variations seen from English sources are the result of a combination of metric illiteracy and rounding.
Some say that gain twist was used to increase barrel life and others to decrease pressure. I suppose with respect to "peak" pressure it might do that, but the act of continuously changing the engraving on the bullet would seem to increase resistance throughout barrel travel, and hence would seen to increase "average Pressure", whereas with standard rifling, once the bullet is engraved, it would "ride the lands" to the muzzle.
Here's a crude video (sorry), in which you can see the change in twist from chamber to muzzle. I'd celebrate a better video being shared.