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Is it possible to slug the barrel of a progressive rifled barrel?
I have 6.5 Vetterli and attempts to slug it come up smooth at .258 with no embossed groove marks.
I'm thinking as I push the slug through the tapering angle changes of the rifling cuts off these groove impressions and makes it smooth?
I hope to cast for this puppy (keeping the loads low.)
Well, I think you've just hit on the reason why gain twist is not such a good idea after all. The first notable person to use it was (I think) Metford, in the days when all bullets were lead. If you look at a soft lead bullet that has been driven through rifling, the lands appear as grooves in the bullet, and you can see that they are at an angle to the longitudinal axis.
Now if the twist becomes tighter as the bullet progresses, that angle becomes steeper. There are two basic options 1) the bullet is actually twisted by the grooves as the angle gets steeper 2) the bullet is partially sheared off by the twist.
The truth is probably a bit of both, depending on bullet hardness. But both effects require considerable force. If you now think of a jacketed bullet, then the force required to either twist or shear the jacket is going to be very much greater than for the comparatively soft bullets of the black powder muzzle-loading period. So a jacketed bullet is going to send up the pressure and cause more wear towards the muzzle than a constant rifling. Of course, the idea of the gain twist was to reduce the stress at the throat end, as the bullet entered the rifling.
So you see, also in ballistics there is no such thing as a free lunch.
I have provided these thoughts with one object in mind: Whatever you do, do NOT try the 0.2675 Hornady bullets in a 6.5mm Vetterli-Vitali, which is a conversion from a black-powder rifle.
I am, by the way, assuming that you are referring to the Model 70/87/16. Italian soldiers disliked the M70/87/16, which was cobbled up as an emergency measure in WWI, and called it "la terribile". The barrel was not a new barrel, but the old barrel was bored out and sleeved, and other parts appear to have been rejects from the standard M91 production.
After WWII the Italians sold them off to US dealser who hyped them up as the "Garibaldi" rifle, a designation which would baffle Italians, as Garibaldi and the Risorgimento were long past in 1916.
This, and a lot more fascinating information, can be found in an excellent book written in German by Wolfgan Riepe, published by VS-Books in Herne, Germany, ISBN 3-932077-30-X or ISBN 978-3-932077-30-2. It's almost worth it just to find out how the Itallians made the carbine conversions with the gain-twist barrels!
Anyway, the good news is that you have hit on the right solution: cast bullets and very mild loads.
Patrick