[ Progressive/Gain Twist Rifling ]
Hi guys, thanks for tuning in to another "Ask Ian" Q&A video on Forgotten Weapons.
I'm Ian, of course, and our question today comes from Jamie on Utreon.
And Jamie ... says, "I know that Naval Gatling guns like the 20mm CIWS use gain twist rifling,
but were there or are there any commercial or military small arms that use gain twist rifling?"
So before we dig into this, first off: the answer is yes.
And before we dig in further let's discuss what gain twist rifling is.
This is also called progressive twist rifling.
Rifling of course are the lands and grooves in the barrel
that cause a bullet to twist as it goes down the barrel.
And typically you have a constant rate of rotation.
So every X inches or millimetres the bullet will make one complete revolution.
In gain twist (or progressive twist) rifling
you have a non-constant rate of rotation for the bullet.
So the bullet will start at a relatively slow twist rate,
and it will end at a relatively fast twist rate.
A good example, the Carcano here starts at a rate of one turn in every 19.25 inches,
and it ends at a rate of one turn in every 8.25 inches.
So the twist rate goes up by about 2.5 times over the length of the barrel.
So why is this done?
There have been a lot of proponents of the system
who claim that it does things like improve accuracy.
It really doesn't. People who have studied this in some depth,
and there are a lot of competition shooters for the past 200 years
who, trust me, have had a lot of spare time and a lot of motivation to study this sort of thing,
gain twist does not give you anything additional in terms of accuracy.
The biggest real effect of gain twist rifling is that
it reduces the initial rotational impact on the bullet.
So you will see this often in larger calibre guns today, Jamie mentioned the 20mm guns,
because ... the diameter at the outside of the bullet is really fairly large.
And if you're trying to spin that from zero to like 3000 rpm instantaneously in essence,
you are putting a lot of stress on the junction between the jacket and the core of the projectile.
And that's actually what led to the system being used
on some military small arms, like this Carcano for example.
So the 1891 Carcano was the first ... rifle adopted by a military to use a 6.5mm bullet.
And that was a fairly long bullet, it had a lot of surface area in contact with the bore.
And the Italians, when they were testing it,
found a lot of trouble with jackets separating from bullets and excessive barrel wear.
And what they determined was that by using a progressive twist
they could more slowly accelerate the bullet up to its final intended rate of revolution.
And then this would make it less likely that the jacket would
basically shear off of the internal core of the bullet.
You've got a copper jacket and a lead core, and the binding between those two elements
is sometimes questionable.
Certainly 1890s Italian-manufactured bullets,
part of the issue they had quite certainly was in fact manufacturing ... quality.
Because the Italians would eventually drop this progressive twist feature in their Carcano rifles.
The ... Model 1941 rifles, which are essentially the same thing as this but a little bit shorter,
used the same 6.5mm cartridge and they have a standard twist barrel.
And it works just fine because bullet production quality had increased substantially.
So that's the big thing that you get is
less immediate shock between the jacket and the core of the projectile.
And as you can imagine if you go to 20mm or 25mm or 30mm projectiles,
that can become much more important, and that's where progressive twist is usually seen.
So what are some of the other systems that have used, or continue to use, progressive twist?
Obviously the Carcano is one, that's the Model 1891 Carcanos.
The rifles and the carbines both were originally developed using it.
Probably the most numerous gain twist guns out there
that people may or may not recognise are Colt revolvers.
So the original Colt percussion revolvers from the Dragoons forward,
the 1848s, '49s, 1851s, 1860s, these were all gain twist barrels.
Now Colt was doing this before a lot of the really serious studies were done
on the true benefits of progressive twist rifling.
... Colt's competitors, like the Remington revolvers,
didn't bother to do this, they just used straight twist rifling.
So it may have been something that Colt was using as an advertising perk,
something that people would think made the guns better.
There's also potentially reason to believe that it ... made the guns easier to clean.
If you had a paper-patched bullet, or if you were just using a solid lead bullet,
spinning it up more slowly deposited less ... lead onto the rifling and thus created less fouling.
These are issues that aren't really relevant to us any more because we're not using black powder.
A couple of others. A guy named Harry Pope was a very famous custom barrel maker
back in the black powder era into the early years of the 1900s.
He made a lot of gain twist barrels.
And ... there's some history of gain twist barrels where there are people who
think that ... gain twist was more accurate because Harry Pope's barrels were gain twist.
The reality is Harry Pope's barrels were more accurate because Harry Pope
spent a lot of time, and a lot of careful measurement and machining
making them very precise and very repeatable.
The fact that he used a gain twist system is sort of ancillary to their performance.
Modern day we still do occasionally see it.
Smith & Wesson's 460 X Frame revolvers actually use a progressive twist.
They start at 1 in 100 and drop to 1 in 20.
And that, ... having not talked to Smith & Wesson, I suspect it is because
there's a wide variety of bullets that people are going to use in those things.
People are going to load those very hot and you actually do have the potential,
because of how high performance, how high velocity, and large bore that cartridge is
that you have the risk of separating the jacket
from the lead core of a poorly constructed bullet that someone's hand loaded, so.
The 500 Smith & Wesson does not use progressive twist rifling, but the 460 does.
And the last thing that I will point out before we shut this video down,
is there is also such a thing as progressive depth rifling.
Where the rate of twist stays the same the whole time,
but the actual depth that the rifling is cut into the barrel changes.
And typically ... it starts shallow at the muzzle and it gets deep at the breech.
And that has some advantages for muzzleloading firearms.
But ... it's really not a thing once you get to breech loading guns.
Anyway, hopefully that answered your question Jamie, this was a fun one
to do some research on to make sure I had all of my ducks in a row before I started it.
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Thanks for watching.