Canadian Airgun Forum

Pellets are fin stabilized, similar to badminton shuttle cork. I would think it requires certain minimum airspeed to have sufficient "steerage". Does it mean pellet needs to stay above certain speed to be accurate?

-TL


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Yes pellets stabilize themselves using their skirt. But without spin their direction can’t be controlled to create groups. Search ‘why do pellets spiral’ as there is some great info there.

Most twist rates are way to fast for pellets ( based of rim/Center fire data) so once the pellets speed to spin rate isn’t optimal it will destabilize.

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Read this article it has some useful data that you can use, especially as a starting point.
https://hardairmagazine.com/ham-columns ... o-pellets/

So, the answer to the question is yes, and the specific velocity depend on the pellet itself. But generally best accuracy will be achieved around 850 fps +- 50 fps for a diabolo pellet. Pellet testing is still necessary and "how good do you shoot" may still be the biggest factor. But for anyone who thinks shooting diabolo pellets at 1000+ fps is a good thing, the data doesn't support it.

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Thanks for the info.

The article is on external ballistics based on point mass model. The projectile is assumed stable in flight (a point mass). I didn't realize the lag time would increase with too high a MV. Interesting. Will dig into it further.

My question is a bit off topic I am afraid. It is about how the pellet is stabilized in flight. The speed here is the instantaneous speed in flight, in stead of muzzle velocity. I think it will stumble when its speed is below certain value that the fin (skirt) can no longer provide enough countering force to keep the projectile head first.

-TL

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Pellets are not Fin stabilzed (like a dart), nor are they Drag stabliized, as is commonly thought.... They are, in fact Flare stabilized.... When a pellet yaws, the flare produces a stabilizing force, but not through drag, but through lift on the opposite side to the yaw.... There is a good description of this at this link....

https://www.gatewaytoairguns.org/GTA/in ... c=169459.0

Pellets require only enough spin to average out imperfections in the pellet, an absolutely perfect pellet, leaving the muzzle with no yaw, would not need spin.... Of course this never happens, hence the need for a bit of spin, but much slower than is typical for airgun barrels.... About 100-200 calibers per turn is all that is required.... ie for a .25 cal, a 25-50" twist rate....

I suppose it is possible for a pellet to be unstable at some velocities but not at others, but the most likely point for that to occur is in the Transonic zone, from Mach 0.8-1.2 (900-1350 fps)....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Flare stabilized? I'm not familiar with that term. Let me look it up. Thanks.

Pellet indeed doesn't need spin. But in practice rifling helps because of exactly the same point I'm making. Upon exiting muzzle, it has negative air speed as the compressed air moves faster. Without adequate positive air speed, I'm afraid flare stabilization doesn't work either.

-TL

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I understand your point, but at best, different parts of the pellet will see different airflow speed and direction, relative to the surface.... The velocity of the air exiting the muzzle cannot exceed the velocity of the pellet, can it, as the air is contained behind the pellet until it exits?.... I think a better analogy of what is happening is uncorking a wine bottle, as soon as it is uncorked, the air pressure behind it spreads in (almost) all directions, and is immediately (for practical purposes) slowed by running into stationary air molecules....

All projectiles exit the muzzle at some angle of yaw (small but there), and once in flight, the flare stability (for a diabolo pellet) or gyroscopic stability (for a bullet) determine how quickly (if at all) that yaw is damped out.... In the case of Dynamic Instability, the yaw increases, and the projectile corkscrews....

I recommend you study the articles by "ballisticboy" at that link, they are written by a (retired) UK Ballistician, Mile Morris, a contemporary of Robert McCoy of the (US) Aberdeen Proving Grounds....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

In powder burners, gas exiting muzzle faster than the projectile. Most certainly similar, if not the same, in airgun.

Assuming perfect gas seal, gas moves at the same speed as the projectile before exiting muzzle. But once that seal is broken when the projectile parts the crown, the residual pressure in the bore will instantly accelerate the gas or air molecules faster than the projectile, creating momentary negative air speed. At the very least, the air speed experienced by the pellet skirt will be much reduced.

I have come across articles written by Mr Morris. Will be reading more. Thanks.

-TL

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You will get no argument from me!.... How far that effect extends from the muzzle, or affects the stability of the pellet, is the question.... Is there some effect, no doubt.... It is one of the factors contributing to the initial yaw, and ultimately to group size, along with a host of others, such as the accuracy and squareness of the crown, and the skirt of of the pellet, whether or not the CG of the pellet is in the center of the bore, and a host of others....

The effect of the flare (when working properly) is to reduce that yaw as the pellet travels to target.... If it doesn't do that properly (for example if the CG of the pellet is in the wrong place, relative to the CP), the pellet can suffer Dynamic Instability, increasing yaw with distance, leading to spiralling.... The tendency toward this is increased as the pellet slows down, because it doesn't lose RPM as fast as it does velocity.... This means that at the target, it arrives in a situation as if it had been shot out of a much higher twist barrel, but at lower velocity (which can often cause Dynamic Instability).... This is why we see pellets shot at a high velocity out of fast twist barrels spiralling past a certain range.... A slower twist rate helps eliminate this annoying (and accuracy destroying) phenomenon....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Again though, it depends on the pellet. I am shooting .25 calibre Hades out of a Condor with supposedly 1:17.71" RH. Got that off the net.
On top of that, the bore is said to be choked.
I am sending them at 1,034fps and they are not spiraling at 118yards, as I am easily hitting 1" to 2" rocks at that range, virtually every shot
on our small bore range.
U-tubers have stated that the Hades pellets are not very accurate past 80 yards. My rifle appears to disagree with that, along with the speed
thing. Perhaps this is simply pellet design and rate of twist differences.
Do pellet skirts spread after leaving the muzzle due to their thin skirts & "muzzle blast" as happens with powder burners shooting hollow based bullets?
Some pellets have much thinner than others.

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Best Wishes
Daryl

I don't think they expand after leaving th emuzzle.... but they sure obturate in the bore, with the back of the skirt expanding.... Again, it depends on the thickness of the skirt, and the hardness of the lead....Here is a photo of some JSBs (thin and soft), fired and unfired....



Fired on the left, unfired on the right (pushed through the barrel)....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

Note also the "heavier" rifling impressions on the fired pellets. VERY interesting, that there is also some obturation of the nose, it appears,
compared with those pellets merely pushed through the bore.
Interesting indeed. Tks Bob.
I still don't understand why my Condor does well with these Pellets in the transonic zone. They are solid 1/2" groupers at 55 yards. I'd have to check
my books, but iirc, they are running either 1,034fps or 1,074fps.

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Best Wishes
Daryl

I could be wrong, but I think both the head and skirt has diameter matching the barrel's land diameter. The high pressure air blows up the skirt and makes it engage the riflings. The head just rides on the lands like a BB.

-TL

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That's correct, except that in Bob's pictures, the pellets that were shot, show heavier rifling marks on the heads, than-do the ones simply pushed through the barrel. That in itself suggests there is some obturation of the pellet noses as well as the base.
BB's are undersize and are not actually guided by the bore (tops of the lands in rifled guns), except that the lands are bumped against as the BB careens down the bore. It's direction at the muzzle is opposite to the last surface of the bore it bounced against.
This is the same action as the very much undersized round balls loaded in 12 bore shotshell ammo back in the early 1900's, called pumpkin(punkin) balls (early slug loads). These were QUITE inaccurate.
In the 1980's I developed loads for my own 12 bore SxS shotgun with round balls that were quite accurate, that is, accurate enough to shoot deer out to 100yards with them.

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Best Wishes
Daryl

Interesting. I can see under-sized pellet head not being good for business. It opens the door to the pellet wobbling down the bore.

Some head obturation is quite likely. There is quite a bit of force pushes on the pellet. The lead is rather soft in pellet. That force will squash the head flat some.

-TL

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