Let’s Take A First Look At Airgun Barrel Twist Rates

Let’s Take A First Look At Airgun Barrel Twist Rates


This is the second post in our series about airgun barrel twist rates.


Airgun Barrel Twist Rate Assumptions​


First I’m going to start this discussion on airgun barrel twist rates with some guesses and assumptions. Are they right or wrong? Somebody will certainly tell me, or we will find out…

Assumption 1. Before we start looking to find data in confirmation, my guess is that one of the most common twist rates for .22 caliber airgun barrels is 1:16. We’ll find out if this is true in future.

Assumption 2. It seems that the “traditional” and most common twist rate for .22 Long Rifle firearms barrels is 1:16.

As .22 LR firearms were in existence well before mass-production airguns, my GUESS is that 1:16 became a common twist rate for .22 caliber airguns because the machinery and tooling already existed and worked well for .22 LR. After all, why mess with success?



So far as I am aware, the earliest Crosman airguns were in .22 caliber. There’s some evidence for my assumption!


Manufacturers Specifications For Airgun Barrel Twist Rates​


OK, so what do manufacturers say about the twist rates in their airgun barrels?

First, let’s start with one of the most prestigious airgun barrel manufacturers in existence. Lothar Walther.

Lothar Walther publishes detailed specifications for its range of “non custom” airgun barrel blanks. Here’s the range:

.177 Caliber Lothar Walther Airgun Barrel Twist Rates

CaliberTwist RateGroovesNotes
.1771:1612
.1771:17.76Polygonal Rifling
.1771:2012
.1771:366

.20 Caliber Lothar Walter Airgun Barrel Twist Rates

CaliberTwist RateGroovesNotes
.201:17.712

.22 Caliber Lothar Walter Airgun Barrel Twist Rates

CaliberTwist RateGroovesNotes
.221:1612
.221:17.712
.221:2012
.221:3012

.25 Caliber Lothar Walter Airgun Barrel Twist Rates

CaliberTwist RateGroovesNotes
.251:17.76Polygonal Rifling
.251:17.710
.251:3010

.30 Caliber Lothar Walter Airgun Barrel Twist Rates

CaliberTwist RateGroovesNotes
.301:166

Now let’s look at the barrel twist rate specifications given by some of the airgun manufacturers at the 2024 IWA OutdoorClassics show.

ManufacturerCalibersTwist RateGrooves
AirMaks.177, .22, .251:17.7N/a
Epic (option 1).177, .22, .251:17.712
Epic (option 2).177, .22, .251:17.76
KalibrGun.177, .22, .25. 301:17.712
KalibrGun (Cricket II Smooth).301:29.912
Macavity.1771:17.712
Macavity.221:21.812
Macavity.251:23.410
Macavity.301:22.26
RTI.1771:17.7N/a
RTI.221:16N/a
RTI.251:20N/a
RTI.301:16N/a

Below we have the twist rates published by Karma Airguns. This applies to the Red Panda and other models.

CaliberTwist Rate - Pellet BarrelTwist Rate - Slug BarrelGrooves
.221:17.71:16N/a
.251:17.71:16N/a
.301:241:16N/a
.361:281:22N/a

Interesting, huh?

In the next part of this investigation, we’ll move-on to make some actual measurements of airgun barrel twist rates for air rifles available to the HAM Team. That will add some “real life” test data to the mix.


The post Let’s Take A First Look At Airgun Barrel Twist Rates appeared first on Hard Air Magazine.
 
i am not expert but the machinery i am aware of could machine any twist rate with in reason so i would not suspect the machinery and tooling that had already existed would have any effect on the given 1 : 16 rate?
i would also suspect these folks, although possible not as compleatly knowledgeabable as sientist today and that is not to say they didn't but let assume they where moderately lacking, where indeed knowing on some leval as to balistic and twist rates.
if nothing elese we know a ball needs a different twist rate then a bullet we know this becuase as i understand it the ball lack the the surface to engauges the twist rate lands and get mosting in a rotational motion where as a bullet has more surface and engauges much easier?
alugh i have no clue as this effect air guns i would venture to say calible would have litttle if anythng to bring to bare on the topic instead i would think;
one the lack of suface inregards to a pellet but also remembering the skirt will come into plat as when smacked with that blast of air in expand and ingauges the lands?
two also a bullet has more sureface it will lack the the effect of skirt expntion the pellet has?
as well in a tradition fire arm such as a .22 the inetial explotion would most likey defore the slug to some degree also pussing i into the riffling?
so to my humble way of thinking a air rifle has to be viewed much differant the a firearm.
a pellt allthough less surface engauges better
a slug althouh more surface i assume does not engauge as well
the ari rifle lack a sifitiant enitional blats to seat a slug
and the twist rate reqiured to stablize a pellet over a slug i would suspect is vastly differant then that of a ball over a slug?
please excuse my spelling.
just some thoughts.
 
Your spelling keeps the rest of us sharp, all good😆. I have recovered some slugs from my pcps and I would say they definitely engaged the grooves! Of course not as powerful a blast as gunpowder makes but DEFINITELY more than enough pressure to flair a lead skirt on a pellet (or skirted slug) to engage a barrel. I was thinking about same thing on the most common .22 twist rate, my guess would have been 1:17!
 

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Twist rates will normally get less as calibre increases. Due to the changes in the moments of inertia and the aerodynamic moments with bullet or pellet size, if you use the same twist rate for different calibres, you will either end up with less spin stability than optimum for the small calibres or more spin stability than is optimum for the larger calibres. This will be particularly so with slugs, but it also appears with the latest work I have done, that the smallest groups all require much the same average gyroscopic stability factors for different sized pellets as well, which can only be achieved by changing twist rate with calibre.

With pellets though, there is the added complication of spiralling and the large loss in speed at long ranges. It appears that optimum twist rates will also change depending on the range you want to shoot, again to try to keep within the narrowband of optimum average stability factor values. It is probably best to optimize for long ranges, as the errors at short ranges are much smaller anyway.
 
I'm not so sure about Pan60's statement about twist rates of old. It is true today, but 100 years ago? I suspect Steve's 'theory' is closer for manufacturers in antiquity.

There are published calculators for determine the requisite twist rates of fire arm calibers, weights, etc. Applying those to sub sonic velocities is a bit of a stretch in my opinion. As I stated previously, the 1:18 twist in my .25 caliber FX is not fast enough when the weather is hot. It also fails in altitudes over 4,500 FoS. Read into this as thin air.

To agree with Pan60, FX could very well manufacture faster twist rates. As I understand it, most of the rates they use (for their current models) were established with the help of Matt Dubber. In comparison to where I live, his abode is that of a flat-lander! Fact is, the last two custom .224 firearms I had made, both used 1:12, not in the common 1:14 factory guns are made in. This was done to partly compensate for the higher altitude (and average temperature) I live within. As Bob Sterne once said... (sic) 'When it comes to sub sonic velocities, anything is possible'.
 
There are published calculators for determine the requisite twist rates of fire arm calibers, weights, etc. Applying those to sub sonic velocities is a bit of a stretch in my opinion. As I stated previously, the 1:18 twist in my .25 caliber FX is not fast enough when the weather is hot. It also fails in altitudes over 4,500 FoS. Read into this as thin air.
I am rather puzzled by this as all other studies which have been carried out over the last hundred years or so show that at higher altitudes, high temperatures with thin air, projectiles require lower twist rates for the same stability compared to low altitudes with cold temperatures. The classic example was the M483 which was developed with a large boat tail for low drag and was fired successfully in hot conditions during development, only to be found to be unstable when fired in Canada at sea level conditions. The solution then was to cut half the boat tail off.

The reason for less twist in low density air is simple, gyroscopic stability is obtained by a balance of the aerodynamic moments, which are proportional to the air density, and the inertial moments which are proportional to the spin rate at a fixed speed. Hence, if the air density is reduced then to get the same level of balance between the different moments the spin (twist) rate needs to be reduced. As I say above, this is not just theory, this is shown by many years of testing and experimentation in all conditions.
 
As I am Miles. According to FX, the standard STX barrel twist (1:24) for their .25 caliber Impact is adequate for slug weights up to 29 grains. Yet, they fly like a shotgun if the velocity isn't over 1,000 FPS. And even then, you get fliers. The heavy slug barrel twist (1:18) stabilizes them well, but only over about 990 FPS depending on the ambient.

Why this flys in the face of 'science', I can only say it is due to sub sonic velocities, as there is nothing else it might be.
 

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