Author Topic: Gain Twist Rifling for Airguns  (Read 169 times)

steveoh

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Gain Twist Rifling for Airguns
« on: March 13, 2020, 06:27:32 AM »
Bob,
I was meandering this morning on the internet and ended up reading an article about the rifle used by Lee Harvey Oswald in the assassination of President Kennedy. The Italian surplus 6.5mm Carcano rifle was mentioned to have used gain twist rifling. Gain twist rifling is where the twist rate gets tighter over the length of the barrel.

I’m curious, any idea if any air rifles use gain twist rifling? Is there enough velocity and or projectile length for gain twist rifling to work well in an Airgun? I will assume the needs are very different in such a scenario when comparing slugs vs pellets.


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rsterne

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Re: Gain Twist Rifling for Airguns
« Reply #1 on: March 13, 2020, 09:28:48 AM »
I personally do not see the advantage to gain twist rifling, and in fact feel that there is a negative to it not commonly discussed.... Consider this drawing of what a bullet would look like after firing....



Let's assume that the rifling starts at zero twist, and increases to whatever value at the muzzle.... The groove initially engraved by the lands would be the red lines.... However, at the muzzle, the groove would look like the blue lines.... The resulting grooves in the bullet would be narrow in the middle and wide at the ends, as shown, or some version of that (the location of the narrowest part may not be in the center as drawn)….

Since the rifling lands are increasing in twist angle, they should still accelerate the bullet spin OK.... However, once the bullet leaves the muzzle, the air no longer flows through the grooves "straight" in relation to them, the red edges of the grooves are meeting the airflow at an angle.... I can't imagine that being good for the drag of the bullet....

One other thing.... Think about a bullet with just two narrow driving bands at the front and back of the part shown above.... Now you no longer have a narrow portion in the middle (the bullet doesn't touch the lands there)…. but the ends still have to get wider as the angle of the rifling changes.... At some point, depending on the number and width of the lands.... they would strip the drive bands completely.... as the blue edge formed by the increasing twist rate catches up to the red edge where the groove in front of it was formed initially....

On top of this, I don't see any reason to use a gain twist in the first place.... The object of rifling is to speed up the spin of the bullet in relationship to its velocity.... A constant angle does that.... There is so little energy lost to spinning up a bullet, I just don't see why we should complicate matters....

Bob
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Alan

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Re: Gain Twist Rifling for Airguns
« Reply #2 on: March 13, 2020, 06:06:06 PM »
If one digs into firearms history a bit....

This is another myth, similar to bullet lubrication. Over the years, other accuracy-inducing attributes have been applied to firearms. I'll give you another one.

Way back when (circa 1945), my father got into lubing bullets with molybdenum disulfide. Considering his job during WWII, I'm not surprised. Later on, I started to use Danzac® (tungsten disulfide) as a lubricant. There is not doubt that lubricants increase barrel life. But considering the expense of coating bullets, the process far exceeded the monies spent rebarreling a worn out barrel. Nowadays, modern steels have proven bullet lubricating really doesn't extend barrel life. Yet, again in the 70s, and again in the 90s, shooters again brought back lubrication. The proves, history does indeed repeate itself.

Fast forward, back in the early 1900s, gain-twist reared its ugly head. If we understood how barrels were made back then, and powders being what they were, gain-twist did indeed offer an advantage as it reduced fouling. But, once more modern propellants were introduced, what little (if any) advantage gain-twist offered, is long since past.

Today, however, we know a lot more than were did back when. If there is but one "modern attribute" which has proven its worth, it's smooth-twist. I can't say if FX was the innovator, as the US military arms manufacturing companies were developing smooth-twist barrels back in the late 70s. Eventually, (large) smooth-bore weapons became the norm, with sabots and/or finning becoming the method of choice for stabilizing projectiles. Nonetheless, I believe FX has proven their methodology works! Perhaps tomorrow, things will be different. Engineering, does indeed, march on!
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rsterne

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Re: Gain Twist Rifling for Airguns
« Reply #3 on: March 13, 2020, 06:27:02 PM »
The original FX Smooth Twist barrels were the ultimate in gain twist.... smoothbore until a few inches from the muzzle, then a 16" twist pressed in to form a combination twisted choke.... The result was the pellet skidding through that section, and picking up enough spin to work with a pellet (but not a slug)….

The current Smooth Twist X barrels have a constant twist rate from breech to muzzle, but it is pressed in from the outside, producing a smoother interior finish.... Nowhere does FX call them a gain twist barrel.... they appear to have abandoned the idea....

Bob
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steveoh

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Re: Gain Twist Rifling for Airguns
« Reply #4 on: March 13, 2020, 08:59:29 PM »
Great responses. I kinda thought the idea of Gain Twist Rifling seemed odd and a bit hopeful at best.   Great stuff to chew on though!
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Alan

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Re: Gain Twist Rifling for Airguns
« Reply #5 on: March 14, 2020, 03:47:48 AM »
For what it is worth...

The local PD tech guy who does bullet comparisons, says it is rather difficult to compare pellets or slugs shot from an X barrel. I've looked at ones I've shot into a water trap using a 20X magnifier, and I can't see any striations to speak of. I'm sure at higher powers you might be able to, but I'll add this; Hybrid slugs, shot into clay all but deteriorate into unidentifiable chunks of lead, with no visible striations one could call a rifling mark.

Maybe I should mention what happens to a pigeon shot with an FX hybrid slug, but it is a bit messy for primetime.
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Chickenthief

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Re: Gain Twist Rifling for Airguns
« Reply #6 on: March 14, 2020, 02:34:27 PM »
The original FX Smooth Twist barrels were the ultimate in gain twist.... smoothbore until a few inches from the muzzle, then a 16" twist pressed in to form a combination twisted choke.... The result was the pellet skidding through that section, and picking up enough spin to work with a pellet (but not a slug)….

The current Smooth Twist X barrels have a constant twist rate from breech to muzzle, but it is pressed in from the outside, producing a smoother interior finish.... Nowhere does FX call them a gain twist barrel.... they appear to have abandoned the idea....

Bob
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Alan

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Re: Gain Twist Rifling for Airguns
« Reply #7 on: March 14, 2020, 03:40:17 PM »
You are of course correct.

What some might call regular cut rifling is actually a bit more modern, but still old dating back nearly 200 years. The first rifling was actually a lot like the FX Smooth-Twist we have today, as there wasn't any defined lands and grooves. They were more like curvilinear is shape. Odd, really... What often seems like new, isn't much more than a rehash of something old!
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