(Grooved rifling on the left, polygonal on the right.)
It’s called “gain twist.”
The .460 smith and wesson magnum has a gain twist. There’s an article in a fairly recent American Rifleman about it.
Keep in mind that a bullet accellerates down the barrel. So even if you have a steady or constant rate of twist, the bullet will experience a progressive rate of twist due to it’s increasing velocity as it travels down the barrel.
Bite your tongue and look up ‘progressive rifling’.
Not in any way associated with liberals/progressives in politics... LOL
There’s at least one specialty manufacturer that seems to use gain twist (aka progressive twist) rifling, here: http://www.sviguns.com/1201.php
The Italian Carcano had gain twist rifling. Nothing terribly wrong with it except it has a tendency to wallow out the groove in the bullet due to changing angle as it moves down the bore.
The advantage is the bullet originally hits the rifling as a slow twist which helps keep the bullet from deforming.
I have no idea why S&W chose it.
Inconceivable!
There are a few people who still make custom black powder barrels with this feature, to gradually increase the spin rate of a soft lead bullet or ball. But, they are talking about increasing spin from 1 in 96 to 1 in 48. These are VERY slow rates, meant for the longer barrels and lower pressures of black powder.
The harder material of copper, when swaged into constant rate rifling, pretty much solves stripping as a problem.
Keep thinking, though, Butch, that's what your good at...
Variable or varying? The former don’t quite seem possible.
I made the rifle for deer and hog. I can start a pure lead round ball off with a heavy charge and the gain twist limits stripping, where the rifling fails to hold on the patch and ball. It also works OK with short conicals. The balls usually stopped under the hide on the offside, just about flattened out. I never had to do any chasing down of wounded critters, it hits hard.
As I remember my barrel starts at 1 turn in 72" and is about 1 turn in 56" at the muzzle.
Jacketed bullets might be a whole different matter, the twists involved are usually much faster.
Since we’re on the subject of Rifling, does anybody have any insight on how exactly are barrels rifled – ie. What type of machine process is used to rifle a barrel?
Broaching?
Milling?
As noted, it’s been done, but it is extra work for a questionable benefit. First, it actually distorts the bullet more than constant twist rifling becuase the grooves in the bullet have to expand to accomodate the grooves in the barrel when the twist turns to a sharper curve.
Second, it takes more work. It’s not so bad now with CNC machines, but before then, the machinist needed a lathe that could change speeds very precisely, and repeat the function exactly each time the tool passed through the barrel.