What's the advantage of gain-twist rifling on a cannon like the Parrott?
Only thing I can think of is that it might allow the expanding skirt more time to engage the rifling before it starts spinning much, decreasing the chance of stripping(?)
In our relatively extensive reading on the subject of rifling, constant twist and gain twist, we have learned a few things. Robert Parrott wrote that his major concern was the shock imparted to those frequently fragile cast-iron shells used in the 1850s and 60s. The gain twist obviously would reduce the torsional forces on the shell at the critical starting point in the bore when the forces of acceleration were also trying to rip it apart. The heavy shell, naval artillery of many nations used it for the same reason. In WWII, the extremely accurate, German 88 used gain twist with no loss of velocity noted.
In a scale Parrott of 1" bore diameter, the advantages would be the same as Harry Pope, maker of legendary Pope rifle barrels noted:
"The advantages for gain twist listed back then where these: The twist being less at the breech gives less friction to the bullet, it therefore starts easier and quicker, giving the powder less time to burn on in front of the chamber, which therefore fouls less than in a barrel of uniform twist at the same necessary muzzle twist/pitch. Also, the slight change in angle of rifling in connection with choke boring, effectually shuts off any escape of gas and prevents gas cutting, which is another cause of imperfect delivery. It holds a muzzle loaded bullet in position much better than a uniform/straight twist barrel.
The above is quoted from the manual of Stevens-Popes barrels literature and remember they are referring to lead bullets.
Now lets fast forward a bunch of years. There are differences in shooting lead bullets vs. jacketed bullets. I'm not claiming I or let alone anyone else I can think of knows all the real answers but I will share some observations.
The gain twist type rifling will what we call will give a mechanical choke on the bullet just like with lead bullets. A lot of bench shooters always ask for the bores of the barrels to be lapped with choke in them (this is another subject). Some comments made to us about the gain twist is that is seems to give a broader tune window (i.e. different bullets and different powders and or charge weights) the barrels seem to be more forgiving." This is all from Frank Green of Bartlein Barrels.
Bartlein barrels have been used by more than 60% of match winning bench rest shooters since 2008. All their barrels are cut-rifled and a large majority are gain twist rifled.
Victor, besides the obvious advantage of a "reduction of stripping", from all of our reading, we have drawn these two conclusions: One, the fact that a mechanical choke is created by forcing the bullet constantly against the driving side of the rifling land reduces gas cutting and blow by. Two, this form of rifling offers an accuracy advantage over button rifled bores by being available only in cut rifled bores. Over 90% of match winning competitive rifle shooters use cut rifled barrels.
Tracy and Mike
