Why do pellets "corkscrew" ?

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Freaking nutation. I've had a couple rifles that did it bad. A pellet will take a nutated path when it's flight is disturbed. It does not get more stable with range like a bullet.

I think anything that disrupts flight will cause it. Gusty tailwind will. I've seen it happen when the sun sets over your shoulder and you can see the flight of the pellet. But some guns just toss them corkscrew with no external forces.

I think it's an uneven crown or a patch of lead or steel disrupting the pellet as it exits the barrel. Just a hunch but it seemed to disappear after I worked on the crown a bit. A super dirty barrel that is tossing strays will do it a lot. A couple patches and it is much less.

Anything can cause it I think. You just have to eliminate the main culprits to find what it is. In my experience it has been some obstruction near the muzzle. A piece of 1000 grit paper rolled into a cone and carefully dressing the crown has helped the most.

Some guys are afraid to even polish the crown and I think that's ridiculous if the crown is rough. If it's a bit out of square polishing won't work and the pellet will always take a nutated path.

I've cut and recrowned a lot of rifle barrels. Polished every one. Have had good luck. I think that would go for airguns doubly. If the pellet is constantly destabilizing over distance it would seem more important than a bullet which is gaining stability over distance. It's just a hunch but it has worked for me in practice.
 
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JimD said:
I would need both the barrel and a gun that could drive that pellet that fast. But Scott provided some interesting information which seems to pretty much prove a lot of speed does not have to lead to spiraling. The other theory I've heard is that cross wind can contribute to a tendency for spiraling. Theory is that the pellet may fly true in low wind but spiral if it encounters a cross wind. It would be interesting to hear what the wind was like in the example Scott provided and the twist rate of the subject barrel.

I do not have stability for pellets resolved in my brain at this point. For "bullets" stability is spin rate versus length of a projectile. Longer bullets have to spin faster to be stabile at long range. Slugs for air rifles are kind of like bullets but they seem to come with cavities on the front and back making them also a bit different. The "hybrid" design is quite different with an unusually large front cavity. But pellets can be at least somewhat stabile in flight without spinning due to their shape. Like a badminton shuttlecock. But we spin them. Seems more complicated. I've seen the stabilizing effect of pellet shape described as drag stabilized. While slugs have a bc that is higher numerically than pellets it is still low by bullet standards. Is that to preserve some drag stabilization?

I also agree with the comments describing spiraling as a reason to try a lower velocity. It seems like you need to shoot at 50 yards or more and film the flight of the pellet so you can slow it down to directly see the spiraling. That makes an investigation a bit more difficult. But given my inability to understand and predict spiraling checking for it seems like the rational choice. I've only shot at 100 yards occasionally. I get groups of 2-3 inches in normal a little bit windy weather. To me that does not suggest I am getting spiraling. I hoped for smaller groups than that but after looking at wind much more closely at my most typical 30 yard shooting distance I am not surprised at my 100 yard group size. A higher bc projectile would undoubtedly help.
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This series of threads may help you to understand pellet stability, which is not what is often described on YouTube as drag stabilized. No gun launched projectiles are drag stabilized, as you would not want to put a parachute on the back of your pellet or bullet.

Pellet Stability

This has appeared on a few forums, mainly in the UK, so some of you may have seen it before. I have tried to make it as simple and none technical as possible. Quite often you will see someone mentioning stability, talking about how pellets go unstable as they slow down or are more accurate...
www.airgunnation.com www.airgunnation.com

Aerodynamic Stability of Pellets

The aerodynamic stability of pellets is a grossly misunderstood subject, mainly it must be said due to misinformation on the internet in videos and articles by "experts". I have previously tried to explain the different types of stability used by pellets. Here I hope to be able to debunk a very...
www.airgunnation.com www.airgunnation.com

Gyroscopic Stability

This is going to be the most technical of the threads on pellet or slug stability. I am going to try to explain how gyroscopic stability works. It is a relatively simple phenomenon, easy to demonstrate in a 3D environment, but not easy to show on a 2D screen. Some people misunderstand what...
www.airgunnation.com www.airgunnation.com

Cross winds can induce spiralling, but you can get spiralling without a cross wind.

All unguided projectiles have to have some spin to give them a low dispersion at longer ranges.
 
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Thanks Ballisticboy. I've read some of your posts before but I just read them again but I will need to read at least once more before they make complete sense. It's late in the day where I am and that is not when I can absorb this sort of information the easiest. I think you are saying that the pellet skirt introduces turbulence that creates a normal force increasing stability if I understood that point. There is always turbulence at the back of an object passing through the air but the void at the back of a pellet increases it and also increases drag, a lot. I don't know if the hollow pellet design is for the airflow effect or to allow the pellet skirt to expand out to seal better against the barrel or both. But I get how the normal force would stabilize the pellet.

The cup that is in the back of some slugs seems too sturdy to provide a "sealing" type benefit but could perhaps affect the airflow enough to be of some stability benefit(?) But it reduces the bc. With all the emphasis on the ballistic benefit of slugs it seems odd to me that they are designed so poorly from an airflow standpoint. I see BCs roughly twice as high for bullets as for slugs of equivalent weight. Materials are different which is possibly part of it but it seems like lead should work at the relatively low under the speed of sound velocity used. Is there a stability reason for this or is it something else? It seems like heavy 22 caliber slugs should have a BC closer to .2 than to .1, for instance.
 
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Today I had nearly zero wind, 78°f, 96% humidity.

My 30 cal Impact is tuned to toss 44.8 grain FX round nose diabolos at about 890 fps muzzle velocity.

I begain the day at 50 yards, then moved to 60 ( POI was just slightly higher than at 50), then 70.

At 70 yards the impacts were on average 1&1/16 inches lower and random, very random, left to right

I was using the military M16A1 sight in target so I have an accurate measure of POA to POI.

Basically there was no grouping so I didn't make any scope adjustments.

Moved back to 60 and all was well again, moved back to 50 and all was even better.

Spiral?!?
 
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Found the issue.

I removed the Sightron scope and reset the rifle to my original MTC SWAT ATOM setup which has the over-bore bipod set much further back.

This ATOM/ Over-bore setup is much more solid than the Sightron/ Over-bore setup.

My 70 yard shots are now fine.

My shot process/marksmanship skill is just not good enough for the Sightron/Over-bore setup.

Once again it was not the rifle or the pellet..... it was me. I am just too ugly for this sport .
 
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daddypaddy said:
my notos started spiraling pellets yesterday and had not done anything to it. just got off work and cleaned the barrel.. super dirty. i’ve been shooting lots of different kinds of slugs and pellets through it for the past 3 weeks lol. anyways, cleaning the barrel seemed to have fixed the problem.
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I was just going to post and say a dirty barrel can also cause spiraling as I've had that on a few barrels in the past. Clean them good and what do you know no more spiraling
 
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This probably does not apply to pellets or spiraling but...

When shooting cast lead bullets in a handgun they begin to lead up around 1000fps. Lead plates itself to the rifling and it's tough to get off. Some alloys do it worse than others. Some rifling designs do it worse than others. A mirror polished barrel does it worse than a barrel with a little tool chatter.

There are obvious differences with airguns. But if the velocity is high enough it's going to lead up. That is going to cause all kinds of issues.

When you get lead particles in the barrel it's no problem. That's not leading. Thats shavings and dust. It just pushes out with a patch.

When it ablates from friction and sticks to the steel it's tough to get out. A badly leaded barrel will often need electrolysis to clean. Bore paste basically just polishes it up. It can get pretty thick.

It's analogous to copper fouling at higher velocity. But it's thicker and stickier and affects accuracy and pressure curves more drastically.

At some point speeds are going to increase enough in PCP rifles that leading is going to be a problem. You guys are going to start shooting lubed or plated slugs like a powderburner. It's speed and friction that causes the leading. Not heat from the propellant burning.

I bet some of you guys pushing a hard lead alloy over 1000 fps. are already experiencing it to some degree. Especially with slugs with no grooves to break up the bearing surface.
 
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