Makes sens, I didn't think at the 'Where would it point". It might be the 3 hours of sleep last night. Thx JimNM
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Why does a pellet destabilize as it loses velocity?
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Yes, because of the "birdie" shape, it will eventually recover to nose on flight. It will not do that predictably and so the end result will still be unacceptable groups.
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oldspook's idea on spin stabilization vs diabolo shape effect seems accurate. I don't buy the concept that the retained higher spin as a pellet slows causes a wobble.
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I think the REAL question isn't "why does a pellet destabilize as it loses velocity", but instead is "DOES A PELLET DESTABILIZE AS IT LOSES VELOCITY?", assuming it was initially stable to start with...
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Certainly a valid question. Seems those who note "groups opening up" would be the ones to ask, especially if they can be certain that it is not a wind related effect and is a large change in group size over a short distance. Not something I've experienced myself but I only shoot my airguns at what would seem to be considered short ranges these days.
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The issue with pellets sometimes becoming unstable and spiraling at a distance is a consequence of degraded dynamic stability from a spin rate which is too high. It is not a controversial thing in the study of external ballistics. Here are a few summary statements and the corresponding links for further reading.
From Miles:
"The change to dynamic instability is due to the increase in pellet spin rate relative to the pellet forward speed as the pellet flies along its trajectory. This apparent increase in spin rate is due to the pellet losing forward speed much quicker than it loses spin until it causes the pellet to become dynamically unstable. It appears to be the dynamic instability produced by the excess spin rate which may lead to apparent spiralling and accuracy effects at longer ranges or at higher speeds. The pellets are still statically stable, in fact the gyroscopic stability has increased, but the dynamic instability is adversely affecting the pellet flight."
https://shooting-the-breeze.com/threads/pellet-stability.44037/
From yrrah:
"It can be seen that the spin rate in revs per unit distance traveled climbed from near 1:19 inches (565 revs/sec) close to the muzzle at 890 fps to 1:12.8 inches ( 480 revs/sec) at 506 fps ( my chronographed velocity @ 200 yards at 4,030 ft adjusted to my farm's elevation 3640 ft) at 200 yards of travel.
This would seem to possibly indicate an excessive static stability condition already by 200 yards."
https://www.tapatalk.com/groups/yellow/pellet-spin-rate-increase-with-range-to-200-yards--t169256.html
From Bob Sterne:
"with pellets, the Cp is well aft, because they are drag stabilized, like an arrow or Badminton shuttlecock.... An aft Cp decreases Dynamic Stablity.... particularly as the pellet slows down (but the RPM doesn't)....
The solution, for pellets, is to use a slower twist, to push the tendency to spiral out beyond useful range.... Tumbling with pellets is almost unheard of, even with extremely slow twist rates.... The FX Smooth Twist barrels only spin a pellet at the equivalent of a 5 to 13 FOOT twist rate, because the pellet strips through the short rifled section at the muzzle, only picking up a slow rotation rate.... Hence, those pellets don't spiral as a result...."
https://www.gatewaytoairguns.org/GTA/index.php?topic=126656.msg1241945
From Miles:
"The change to dynamic instability is due to the increase in pellet spin rate relative to the pellet forward speed as the pellet flies along its trajectory. This apparent increase in spin rate is due to the pellet losing forward speed much quicker than it loses spin until it causes the pellet to become dynamically unstable. It appears to be the dynamic instability produced by the excess spin rate which may lead to apparent spiralling and accuracy effects at longer ranges or at higher speeds. The pellets are still statically stable, in fact the gyroscopic stability has increased, but the dynamic instability is adversely affecting the pellet flight."
https://shooting-the-breeze.com/threads/pellet-stability.44037/
From yrrah:
"It can be seen that the spin rate in revs per unit distance traveled climbed from near 1:19 inches (565 revs/sec) close to the muzzle at 890 fps to 1:12.8 inches ( 480 revs/sec) at 506 fps ( my chronographed velocity @ 200 yards at 4,030 ft adjusted to my farm's elevation 3640 ft) at 200 yards of travel.
This would seem to possibly indicate an excessive static stability condition already by 200 yards."
https://www.tapatalk.com/groups/yellow/pellet-spin-rate-increase-with-range-to-200-yards--t169256.html
From Bob Sterne:
"with pellets, the Cp is well aft, because they are drag stabilized, like an arrow or Badminton shuttlecock.... An aft Cp decreases Dynamic Stablity.... particularly as the pellet slows down (but the RPM doesn't)....
The solution, for pellets, is to use a slower twist, to push the tendency to spiral out beyond useful range.... Tumbling with pellets is almost unheard of, even with extremely slow twist rates.... The FX Smooth Twist barrels only spin a pellet at the equivalent of a 5 to 13 FOOT twist rate, because the pellet strips through the short rifled section at the muzzle, only picking up a slow rotation rate.... Hence, those pellets don't spiral as a result...."
https://www.gatewaytoairguns.org/GTA/index.php?topic=126656.msg1241945
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I think Matt Dubber said in his fx slug liner video, that the heavy pellets somehow becomes a badly designed "slug". If you look at the shape of the 25 grain .22 JSB pellet, there is hardly any skirt left. The midle section of the pellet is thicker than a 16, or 18 grain, as it is filled with more lead. The angle of the skirt is flatter. So the self stabilising force is less present, as there is less drag the skirt can make. To have enough self stabilising force the heavy pellets needs to travel faster than the lighter ones to keep enough drag.
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"It is not a controversial thing". Seems like I see "appears" and "may" (Miles) and "seems" (yrrah) in the above. I'll call it less than settled but you call it whatever you want.
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Above you stated "I don't buy the concept" out of hand. No contrary theories, experiments, evidence, or sources presented. Thank you for registering your dismissal of 3 of the great minds of our online community.
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This is the reasoning I base my opinion on. "I don't buy the concept" is my opinion. Others like to state things as incontroversial fact that I view as opinion.
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My Opinion..
A pellet does not become unstable when it reaches a certain distance. Pellets that are unstable are wobbling from launch. The wobble just becomes more pronounced the further it travels. At higher velocity it can "self-correct" because of the shuttlecock shape. Once it drops below a certain velocity, it's correcting at a slower rate because the drag is lessening. When it reaches this point, the drag starts to increase again, instead of decreasing, because a projectile not flying perfectly straight presents more surface area to the direction of travel.
In archery, there is a phenomenon called parachute drag. If an arrow has too much drag, it doesn't really matter at close distances when the velocity can overcome the effects of the drag. Actually more drag means more correction...which translates to better groups, BUT only at close range. After a certain distance the arrow becomes more susceptible to wind and imperfect flight because of the excessive drag. When this happens, groups open up significantly. The size or the offset of the vane or feather is what creates the excessive drag.
I think of diabolo shaped pellets like an arrow fletched with large feathers and slugs as arrows fletched with small vanes. Vanes are made of flexible plastic, so they have smooth surfaces with less drag.
Just my 2 cents.
A pellet does not become unstable when it reaches a certain distance. Pellets that are unstable are wobbling from launch. The wobble just becomes more pronounced the further it travels. At higher velocity it can "self-correct" because of the shuttlecock shape. Once it drops below a certain velocity, it's correcting at a slower rate because the drag is lessening. When it reaches this point, the drag starts to increase again, instead of decreasing, because a projectile not flying perfectly straight presents more surface area to the direction of travel.
In archery, there is a phenomenon called parachute drag. If an arrow has too much drag, it doesn't really matter at close distances when the velocity can overcome the effects of the drag. Actually more drag means more correction...which translates to better groups, BUT only at close range. After a certain distance the arrow becomes more susceptible to wind and imperfect flight because of the excessive drag. When this happens, groups open up significantly. The size or the offset of the vane or feather is what creates the excessive drag.
I think of diabolo shaped pellets like an arrow fletched with large feathers and slugs as arrows fletched with small vanes. Vanes are made of flexible plastic, so they have smooth surfaces with less drag.
Just my 2 cents.
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I think this situation is different than some of the other recent arguments that either have never been proven (groups at 50 yards getting better at 100 yards), or have incontrovertible proof (BC worsens above 950 FPS and wind performance suffers). In this case we’re in the middle. It is and has been observed and documented that certain pellets at certain velocities and spin rates become unstable downrange.
So let’s look at this from a physics standpoint.
1. The pellet is drag stabilized. Therefore there is an aerodynamic flow of air over the pellets surface as it flies. These forces keep the pellet stable.
2. The pellet needs at least some spin to remain stable as it exits the muzzle due to high pressure air exiting behind it.
3. The spin rate relative to speed, or as a percentage of speed, increases as the pellet travels downrange.
4. It is theorized that the reason some pellets lose stability downrange is because the spin to speed ratio becomes too high for that specific pellet.
Is this due to non perfectly balanced pellets? Does it happen consistently for that specific pellet? Or just some of the specific pellets from a tin? It has also been reported that this never happens with the FX ST barrels because they give the minimum amount of spin required for stability.
Hopefully I summarized what has been discussed so far. I’m not of any specific theory, but I do believe that certain pellets out of certain barrels at certain speeds can become unstable downrange. I personally haven’t observed this but Yrrah and Bob Sterne are truly two of the most knowledgeable airgun experts of our generation.
So let’s look at this from a physics standpoint.
1. The pellet is drag stabilized. Therefore there is an aerodynamic flow of air over the pellets surface as it flies. These forces keep the pellet stable.
2. The pellet needs at least some spin to remain stable as it exits the muzzle due to high pressure air exiting behind it.
3. The spin rate relative to speed, or as a percentage of speed, increases as the pellet travels downrange.
4. It is theorized that the reason some pellets lose stability downrange is because the spin to speed ratio becomes too high for that specific pellet.
Is this due to non perfectly balanced pellets? Does it happen consistently for that specific pellet? Or just some of the specific pellets from a tin? It has also been reported that this never happens with the FX ST barrels because they give the minimum amount of spin required for stability.
Hopefully I summarized what has been discussed so far. I’m not of any specific theory, but I do believe that certain pellets out of certain barrels at certain speeds can become unstable downrange. I personally haven’t observed this but Yrrah and Bob Sterne are truly two of the most knowledgeable airgun experts of our generation.
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Well stated. Particularly, "In this case, we're in the middle".
I can accept that it MAY eventually be proven that the relatively higher spin rate as velocity decays DOES cause the issue. I don't believe that has been proven now so nothing seems to be "the fact". It is absolute statements of such that I am concerned with. It seems that we only have a theory as to the mechanism of action underlying the effect at this point. And as I pointed out previously, Yrrah used terms such as "seems" and "may" an awful lot. I haven't read anywhere where he stated "this is the way it is". Yes, their work is exemplary but I believe it still should be stated as "seems" and "may" because it is still a theory. It is just the best model pellet shooters have to use. And I didn't invoke their names myself.
As to the FX barrels (relatively much lower spin rate as I understand it), does the phenomenon "never happen" or does it happen much less often. If the answer is "never happens" then that would certainly seem to support the spin as cause theory. If it happens "much less often" then other factors would seem to have to be the cause when it does happen. I don't have an FX but I would love to know the answer.
After this "discussion" started I read as many different opinions of the various researchers as I could find. One stated he could create instability by slightly damaging the pellet head prior to shooting. Such is why I don't believe that one can state "it is the spin".
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I honestly read initial post & 1st 2 answers.
I think the new redesigned or MKII that have skirts sized to exactly same as head have pros & cons.
I do think different twists play a role. I think distances a buyer expects to shoot at most often requires a dedicated set up. I’ll say this then read on and see.
If you size the skirts to the head you’re going to loose some of The Diabolo pellets best feature which IS self stabilization from a flared skirt that acts as a badminton shuttlecock.
You all can’t believe that ONE barrel & ONE twist rate will shoot all pellets & slugs as well as the other, if at all. Let me read on and learn. I say redesigned & MKII are for competing set ups at 50-100 yards. High speed yaw and at what distance a given design will do best takes testing.
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"You all can’t believe that ONE barrel & ONE twist rate will shoot all pellets & slugs as well as the other, if at all. Let me read on and learn. I say redesigned & MKII are for competing set ups at 50-100 yards. High speed yaw and at what distance a given design will do best takes testing."
Agree totally. Seems some guns will shoot almost anything and some will be pretty picky about their pellets. Is it twist rate or harmonics or some other factor?
" If you size the skirts to the head you’re going to loose some of The Diabolo pellets best feature which IS self stabilization from a flared skirt that acts as a badminton shuttlecock."
This part I'm not so sure about. Seems it is desirable to have the skirt slightly flared for sealing in the breech (but many say air pressure will do that, something I'm not sure about for maximum efficiency/accuracy) but once fired it seems the muzzle will shape the pellet so that the skirt is roughly the same size as the head. Is the stabilization caused by the skirt being wider than the head or by the wasp-waist shape?
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When the SO point dips downwards the yaw will increase.
I’ve seen too fast cause barrel rolls. I’ve seen slugs and heavy pellets that are only going 750-850 do great at 25-50-100-200. Past 200 I’ve only shot pellets and slugs ONCE. What was weird was I had original 33.95’s set for 900fps and they were amazing from muzzle to 200 yards. We cranked up hammer to throw 31.9 RBT’s @ 940 fps. Never re zeroed. It had been dead on at 75 yards with 33.95 at 900.
Got out to 316 yards and started according to Chairgun, Kestrel & mountain of shale that showed strikes. Mountain had to have been 40-60’ high and we shot from a hill 316 yards away though target was a slight bit higher than our position. I tried but couldn’t see ONE strike of 33.95’s. My guess is too fast, destabilized all over the place. Possible lots more drop and not enough power to show visual strike. The slugs on other hand were still doing great. I only had a few but I did manage to see many close hits and one actual hit. Using hold off & over with single shot and about 15 slugs to test, I’m surprised I hit it at all let alone came close.
I learned pellets are capable of targets to 200 in that rig. Maybe further but slugs definitely took hold of the wheel somewhere between 200 & our tested 316.
I’ve seen too fast cause barrel rolls. I’ve seen slugs and heavy pellets that are only going 750-850 do great at 25-50-100-200. Past 200 I’ve only shot pellets and slugs ONCE. What was weird was I had original 33.95’s set for 900fps and they were amazing from muzzle to 200 yards. We cranked up hammer to throw 31.9 RBT’s @ 940 fps. Never re zeroed. It had been dead on at 75 yards with 33.95 at 900.
Got out to 316 yards and started according to Chairgun, Kestrel & mountain of shale that showed strikes. Mountain had to have been 40-60’ high and we shot from a hill 316 yards away though target was a slight bit higher than our position. I tried but couldn’t see ONE strike of 33.95’s. My guess is too fast, destabilized all over the place. Possible lots more drop and not enough power to show visual strike. The slugs on other hand were still doing great. I only had a few but I did manage to see many close hits and one actual hit. Using hold off & over with single shot and about 15 slugs to test, I’m surprised I hit it at all let alone came close.
I learned pellets are capable of targets to 200 in that rig. Maybe further but slugs definitely took hold of the wheel somewhere between 200 & our tested 316.
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Okay I’ll bite 
I don’t see how the damaged head example serves as a counterpoint. By damaging the head of the pellet, a deliberate imbalance has been created which negatively affects the pellet’s stability. In so doing, the pellet is now simply more sensitive to developing a spiral than it would have been if it were in its original undamaged state.
Consider for a moment if this damage/defect were shaped like a small fin, like a little piece of mold flashing on the head at the parting line, present only on one side of the pellet. If this pellet were fired with no spin, it would promptly veer off course. But by applying a spin, the gyroscopic effect is able to tame its tendency to veer off course, but the imbalance is still present and spinning rapidly which causes precession and nutation. Near the muzzle, the pellet is moving at high velocity so there is a correspondingly high drag force produced by the shuttlecock geometry pulling at the skirt which helps keep the nose leading in the forward direction. However as the pellet slows at a distance, this otherwise helpful drag force diminishes and the precession and nutation begin to take over. If bad enough, a noticeable spiral may develop.
Note this defect can be either geometric (e.g. damage) or gravimetric (weight imbalance) or both. And indeed, in practice even our best quality pellets have a little of both. So in the case of an undamaged pellet, it too may begin to spiral, just not as soon or as severely.
To be perfectly clear, the spin rate is not the cause of the instability. It simply influences the extent to which these inherent defects lead to the pellet becoming unstable.
BTW, I also think it is a unrealistically high burden of proof to require an FX barrel to _never_ produce a spiral in order to be convinced of the effect of having too high a spin rate. How would that work? Do we assume a STX liner with a 1:40 twist rate represents the magic number? Or the 1:60 to 1:150 range of the original ST? Or that all diabolo pellets exhibit stability to the same extent despite differences in geometry and manufacturing quality, such that one would have an expectation to take any of these pellets at random and check for the presence or absence of spiraling, and then use that result to conclude whether excessive spin rate is the cause of a loss of stability at a distance?
Lastly, I re-read my initial post to this thread and I do want to apologize for having represented spin rate as THE cause. As in the only cause. I do not know that to be the case and did not intentionally present it that way but I can see that anyone would reasonably interpret it that way. With that said, I do think it is the primary cause. I cited a few sources and Centercut subsequently gave a tip of the hat to yrrah and Bob, both of whom have done great work and are probably recognizable to most folks who have been a part of this online community for a while. However I wanted to point out that I regard Miles as the most credible source for this particular subject. He is an engineer with a vast knowledge of ballistics and aerodynamics so, yes, I am willing to accept his expertise on this matter. Not to the extent that I take it all uncritically, but when it agrees not only with the literature on the topic of dynamic stability but also the observable behavior we see in the world, that’s good enough for me.
I don’t see how the damaged head example serves as a counterpoint. By damaging the head of the pellet, a deliberate imbalance has been created which negatively affects the pellet’s stability. In so doing, the pellet is now simply more sensitive to developing a spiral than it would have been if it were in its original undamaged state. Consider for a moment if this damage/defect were shaped like a small fin, like a little piece of mold flashing on the head at the parting line, present only on one side of the pellet. If this pellet were fired with no spin, it would promptly veer off course. But by applying a spin, the gyroscopic effect is able to tame its tendency to veer off course, but the imbalance is still present and spinning rapidly which causes precession and nutation. Near the muzzle, the pellet is moving at high velocity so there is a correspondingly high drag force produced by the shuttlecock geometry pulling at the skirt which helps keep the nose leading in the forward direction. However as the pellet slows at a distance, this otherwise helpful drag force diminishes and the precession and nutation begin to take over. If bad enough, a noticeable spiral may develop.
Note this defect can be either geometric (e.g. damage) or gravimetric (weight imbalance) or both. And indeed, in practice even our best quality pellets have a little of both. So in the case of an undamaged pellet, it too may begin to spiral, just not as soon or as severely.
To be perfectly clear, the spin rate is not the cause of the instability. It simply influences the extent to which these inherent defects lead to the pellet becoming unstable.
BTW, I also think it is a unrealistically high burden of proof to require an FX barrel to _never_ produce a spiral in order to be convinced of the effect of having too high a spin rate. How would that work? Do we assume a STX liner with a 1:40 twist rate represents the magic number? Or the 1:60 to 1:150 range of the original ST? Or that all diabolo pellets exhibit stability to the same extent despite differences in geometry and manufacturing quality, such that one would have an expectation to take any of these pellets at random and check for the presence or absence of spiraling, and then use that result to conclude whether excessive spin rate is the cause of a loss of stability at a distance?
Lastly, I re-read my initial post to this thread and I do want to apologize for having represented spin rate as THE cause. As in the only cause. I do not know that to be the case and did not intentionally present it that way but I can see that anyone would reasonably interpret it that way. With that said, I do think it is the primary cause. I cited a few sources and Centercut subsequently gave a tip of the hat to yrrah and Bob, both of whom have done great work and are probably recognizable to most folks who have been a part of this online community for a while. However I wanted to point out that I regard Miles as the most credible source for this particular subject. He is an engineer with a vast knowledge of ballistics and aerodynamics so, yes, I am willing to accept his expertise on this matter. Not to the extent that I take it all uncritically, but when it agrees not only with the literature on the topic of dynamic stability but also the observable behavior we see in the world, that’s good enough for me.
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