[voodoochild]

Kind of a technical question, but...

I understand that a barrel has to have a sufficient twist ratio in order to stabilize bullets in flight. Other than a bullet coming apart and fragmenting mid-flight, how is a bullet "over-stabilized".

Example... Let’s say one 308W bullet stabilized with a 1-10 Twist, and another is stabilized with a 1-12 twist. Why would it hurt stabilization and accuracy if you lowered the twist of both rounds as long as they don’t fragment mid-flight?

My understanding is that different gr bullets need different twist ratios due to their length. And it makes sense. But it doesn’t make sense that a bullet spinning too fast can be less accurate. (As long as it doesn’t come apart)

 

[sionaprhys]

The bullet needs to turn its nose into the air so that it continues to fly point first. This is often referred to as the bullet “going to sleep”. The aerodynamic shape will cause this unless it's over stabilized.

If a bullet leaves the muzzle with a tiny bit of yaw (and they almost all do) and it's spinning too fast to aerodynamically correct its flight, you will lose accuracy and lower the ballistic coefficient as it's not presenting the most effecient frontal shape to the oncoming air.

At longer ranges, generally past 300 yards, the bullet needs to turn nose down as the flight path begins to curve down so that it continues to fly point first. An over spun bullet will resist turning into the flight path and the nose will remain turned upward relative to the ballistic curve. Once again, the less efficient shape presented to the oncoming air will lower the ballistic coefficient making the bullet lose velocity faster and more prone to drift.

You probably won’t notice an over stabilized bullet unless you’re shooting competitive benchrest or shooting out past 300 yards.

It's not just length but also aerodynamic profile that determines how much spin a bullet needs to properly stabilize.

 

[MZ5]

Voodoochild, you might be interested in some of the articles written by Bryan Litz, who is now the ballistician for Berger Bullets. You can find some of them at:
http://www.appliedballisticsllc.com/index_files/Articles.htm
The ones there are freely readable, and I believe that at least one directly addresses your question. The way I recall it, he says something fairly close to 'don't worry about it,' but perhaps I'm not remembering properly.

 

[Davers]

Kind of a technical question, but...

I understand that a barrel has to have a sufficient twist ratio in order to stabilize bullets in flight. Other than a bullet coming apart and fragmenting mid-flight, how is a bullet "over-stabilized".

Example... Let’s say one 308W bullet stabilized with a 1-10 Twist, and another is stabilized with a 1-12 twist. Why would it hurt stabilization and accuracy if you lowered the twist of both rounds as long as they don’t fragment mid-flight?

My understanding is that different gr bullets need different twist ratios due to their length. And it makes sense. But it doesn’t make sense that a bullet spinning too fast can be less accurate. (As long as it doesn’t come apart)

Depends on barrel length vs. rate-of-twist. If your .308's barrel is 20" or less, then a 1-in-10" twist is better. If your barrel length is 22" or longer then a 1-in-12" twist is okay.

 

[MikeG]

I believe the phenomenon referred to above is called spherical predecession (if I got the spelling right), and was the subject in a Precision Shooting magazine or two. Yes, too much spin does cause issues with a highly arching bullet because the nose does not point back down at the angle it should when the bullet heads back to earth. In the example that the magazine cited, it was 155gr. Palma match bullets from a .308 at 1,000 yards if memory serves. 1-13" twist rate is I think correct for that bullet. With a faster twist there are problems at the 1,000 yard line.

The slight difference in twist rates could cause a bullet to stay out of the transonic zone at 1,000 yards. While the example cited was for a full 1,000 yards, the distance was less important than the fact that the bullet was starting to drop to near the speed of sound.

So it could occur at lesser ranges, or greater. But with modern bottleneck rifle cartridges and spitzer bullets, it's ordinarily going to be beyond 300 yards and most likely well over 600 yards. Definitely to be ignored at typical hunting ranges.

 

[unclenick]

Barrel length is not really a very significant factor in stability. I ran some numbers: For a 168 grain MatchKing at 2550 fps (20" tube) and 2600 fps (22" tube), that 2" barrel length increase only changes the gyroscopic stability factor (s) of a 12" twist fröm 1.708 to 1.719. If, instead, you change the twist of the 20" tube to 10, then the stability factor goes fröm s=1.708 to s=2.460. Like bullet length, twist is a big player in stability, where different bullet weight (for a fixed bullet length) is a middling factor, and velocity for a given rate of twist is the smallest factor.

An important aspect of the over-stabilization issue, in addition to the extra drag due to yawing nose up and the lift it creates that bleeds off energy, is bullet mass symmetry about its spin axis. If the bullet has any imperfection in that mass distribution, the spin is eccentric and wobbles around the line of the trajectory. How much it wobbles depends on the size of the mass distribution error and the rate of spin. The faster it spins, the greater the wobble for a given flaw. Assuming wobble isn't extreme enough to cause the bullet to become totally unstable, it still introduces both added drag and small random deviations in the trajectory (drag noise) that open the groups up at the target. For this reason, better jacket and core symmetry is one of the elements of superior accuracy achieved by match bullet construction.

At the other end of the spectrum is too little spin. Any bullet fired straight with a stability factor of s>1.0 will not become unstable and tumble, but as Sionaphrys said, bullets launch with some degree of yaw, introduced in large part by muzzle blast passing the bullet and pushing on its base. This yaw is not exactly the same, round to round. It is made much greater if either the bullet base or the muzzle crown are not perfectly symmetrical. The yaw and spin combine to rotate the bullet spin axis around its center of gravity and therefore around the trajectory path. This is fast precession (coning motion) and nutation. If s>1.0, this motions irons out and the bullet nose ceases coning and nutating, but it takes time and distance down range to get there. The nearer s is to 1.0, the longer it takes. If s<1.0, instead of ironing out, the circles get bigger and bigger until the bullet is overturned by air pressure and starts to tumble. If s=1.0 exactly, the coning never gets bigger or smaller, so the bullet never "goes to sleep".

You can see the initial helical corkscrew due to coning in the air disturbance caused by a bullet (its wake). At Camp Perry at the 600 yard line and further back that's easy to do standing behind the firing line because you can position yourself to see the bullet wake arc upward against the sky. Harold Vaughn has some computer plots of the nose spiraling inward over distance for different values of s.

In any event, the two extremes of error source are what you are trying to balance when you choose a target value of s by selecting a twist rate for any given bullet. The match bullets are so well made any more that they often will still shoot well at values of s=3 or sometimes greater. If you are benchrest shooting, Harold Vaughn thinks s=1.4 is about the best compromise. Don Miller thinks 1.5 is best. I've seen a best compromise number suggested as high as 1.7, but it really does depend on how well the bullet is made, not to mention assuming a perfect crown on the barrel and a perfect barrel time load for minimum shot-to-shot muzzle disturbance.

If you are missing any one of the above elements, you'll find the best s value changes. For example, a barrel with a slightly eccentric crown will likely do better with a shorter bullet that gets a higher s value fröm its rate of twist. That's because the crown imperfection makes greater muzzle blast induced yaw to recover fröm by the time the bullet gets to the target, so it needs to go to sleep faster. In this situation you will also likely find a faster powder that doesn't push the bullet quite as fast, but that produces less muzzle blast also produces better accuracy. Flat bullet bases that spend less time clearing the muzzle will also be easier to shoot accurately fröm such a gun.

 

[voodoochild]

Okay, so imperfections in the bullet are amplified more the faster it spins. Makes sense.

What I am now having questions about is bullets pointing nose down at longer ranges. The Marine Corps has always taught me that a bullet stays at the same angel as it’s fired. And it sounds like a few of you are suggesting that a bullets nose drops at longer ranges and enters the target the same as it would 100m. So if you fellas could clarify if you are saying that of not. And if so, how does that work.

by the way, thanks for the to the articles MZ5.

 

[sionaprhys]

Because the 1 in 7" twist of the M16A2 over stabilizes the 63 grain M855 round.

 

[Davers]

One fact, I am certain of, is if one uses 55 grain bullets in a slow twist .22 Caliber HP rifle with a 1-in-16" twist, the accuracy & stability will not be as good as if fired from a 1-in-14" or 12" twist. The velocity must be high enought for the bullet to develop enough momentum to stabolize the heavier bullet.

 

[MikeG]

Okay, so imperfections in the bullet are amplified more the faster it spins. Makes sense.

What I am now having questions about is bullets pointing nose down at longer ranges. The Marine Corps has always taught me that a bullet stays at the same angel as it’s fired. And it sounds like a few of you are suggesting that a bullets nose drops at longer ranges and enters the target the same as it would 100m. So if you fellas could clarify if you are saying that of not. And if so, how does that work.

by the way, thanks for the to the articles MZ5.

If that was universally true for all projectiles, you couldn't hit anything over the horizon with artillery. Think about high-angle projectiles (mortar rounds, howitzers, etc.). If the rate of spin in the projectile is correct, then the nose comes over at the correct angle on it's way down. If not.... then yes, you run into problems.

Rifle bullets are no inherently different than cannon shells in their flight characteristics.

 

[MZ5]

Voodoochild, the Marine Corps instructed you correctly, at least as far as practical application to modern/current infantry riflery.

First of all, remember that spin stability increases as a bullet travels downrange. Next, have a look at the "Spin and Coriolis Drift" article on the page I linked. Note that if a bullet 'nosed over' vertically in precise alignment with its velocity vector (precise direction of travel at any given instant) all the way through its flight path, it would also turn its nose to the right or left, depending upon whether your rifling is right-hand or left-hand twist. Because gyroscopic stability increases as the bullet travels further downrange, this nosing left or right would get worse and worse as the bullet noses over further and further downrange. So, you'd lose aerodynamic efficiency for that reason even as you were 'gaining' it by having the bullet 'nose over.'

Litz has written/shown (I think in one of his articles on the linked page, but perhaps I saw it elsewhere?) that the impact on trajectory resulting from the amount of increase in aerodynamic drag on a rifle bullet from the bullet failing to nose over, is quite tiny indeed.

I'd think about bullet mass symmetry as mentioned above, but I wouldn't bother with the issue of nosing over, especially since there are countervailing consequences as the bullet does so.

 

[MikeG]

As a practical matter yes, but the instructor was wrong, to be blunt. If bullet noses stayed pointed at the same angle up it would be nigh impossible to hit targets at very long range. Forget about spitzers - have you seen long range black powder shooting? Those trajectories are quite arched indeed. They would soon be tumbling and going every which way if the nose stayed up, say at 5 degrees, as the bullet was coming down at 10 or 15 degrees. Yet those shooters can hit stuff 1,000, 2,000 yards, and beyond. Not easy but possible.

The bullet nose does not necessarily point back down at the EXACT angle that the bullet is headed to earth at. If I read the article correctly, it lags the angle somewhat. The amount of lag is the issue. Too much difference (caused by too much spin on the bullet), and then the drag goes up. It is true that the effect it minor inside of 1,000 yards, but it does exist and proves the point. Otherwise it would matter not in the least what the twist rate was when shooting a the 1,000 yard mark. Sorry but I forget the exact mechanics of how the nose gets pointed down to follow the trajectory.

It was interesting that the author was not drawing on experience from ballistics or the shooting sports, but knowledge of orbital flight paths to figure this out. Subtle stuff indeed and not Riflery 101 for sure.

For specific cases, at specific ranges, the effect of all of this may not matter one bit. A bullet spun too fast (standard weight in a non-standard twist, ie, 55gr. in a 1-7" .223 barrel) will keep the nose up excessively. Even then, the nose will start to be pointed down along the bullet path. Just not an ideal amount. But to say that the bullet will keep the exact nose angle all the way to the target is not correct except at extremely short ranges. Once it goes to sleep then the nose does not vary right or left much, but will in fact start to pitch over, slightly, as the trajectory arches down.

For the average Marine Corp drill instructor, giving information to the average recruit, whether the answer is correct or not is of no practical consequence. He could have told his recruits that the bullets spin end over and and you could still go out and shoot the qualification course. You have to be in a long-range competitive shooting sport for it to matter. Even then the shooters may not know or care. They just use what works.

Sorry if this comes across as nit-picking.

 

[MZ5]

Not at all, MikeG. I think it's interesting to discuss how the specific technicalities bear upon the reality. My point was that the specific technicalities don't appear to bear on reality too much for small arms, and to the extent they do, there are additional and sometimes countervailing specific technicalities to consider.

I'd not be too quick to say that the author's (Bryan Litz) comments are not drawn from shooting experience. He's a national champion competitive shooter and the current head ballistician for Berger Bullets. He formerly worked in this field for the military as well. So I think he has significant practical experience to augment his formal and theoretical education/experience.

Again, a 'technical' point to remember is that nosing a spin-stabilized bullet over causes it to nose sideways at the same time. So, it's something of a trade-off because vertical nosing over gains you a minimal amount of aerodynamic improvement, which is counteracted by a minimal amount of aerodynamic decline from the nosing sideways. I believe that unclenick has also posted here before how bullets actually run downrange at a very small up-and-sideways angle due to these forces? Please correct me if I'm mistaken about that.

The long-range shooting you mention might be a bit misleading? The bullets don't tumble because they can't. If the bullets are stable when they leave the barrel, and so long as there's no trans-sonic disruption severe enough to cause tumbling, then they become more and more gyroscopically stable the further downrange they get. So they'd never be able to tumble; they're too well stabilized. Failure to nose over, or to nose over completely, simply reduces BC a touch, and thus the trajectory is slightly worse than our ballistics programs tell us. Litz addresses this point in one of his freely-readable papers, though perhaps it's not at the exact link I provided above.

You can account for all these various forces, attack angle vs. trajectory discrepancies, and flight path effects, and the military does exactly that (mostly with larger guns and projectiles). It's just that there may not be enough reason to calculate all that out for small arms? We make corrections for these and other factors via sighters, etc., but we don't crunch the actual numbers except as a hobby or educational exercise (which can still be lots of fun! )

 

[unclenick]

As Homer Powley (I think it was) put it, the aerodynamic forces "conspire" to point the bullet into the trajectory path. There is a constant translation of overturning forces to the opposite side by spin that makes this true. It is why a bullet yawing most extremely after clearing the muzzle has its nose describe a helical path around the mean trajectory path (the coning or precession I mentioned earlier). The degree to which the bullet resists that aerodynamic conspiracy is dependent on how fast it spins, its mass, and how hard the air resistance reaction force is that it experiences. The degree of success they have in resisting the conspiracy is indicated by how far their average nose-up yaw above the line tangent to the trajectory at any given moment in flight. That is a function of the gyroscopic stability factor which takes spin, velocity, atmospheric density, bullet length and mass all into account in one number.

The tendency to nose over does, in fact, occur. Just look at the angled ramp on the back of a Springfield '03 ladder sight to see how much the sight windage has to move to the side to compensate over a long distance. Typically, with normal gyroscopic stability factors, you see around one foot of windage drift at 1000 yards due to this effect.

If you want to calculate change in stability with distance, you can. Velocity loss is in any ballistic tables. Loss of spin is slower (which is why s increases with distance) but there is surface friction slowing it. Geoffry Kolbe has a spin decay approximation of:

Where current spin rate is N, and the initial spin rates is Nm (spin rate units are your choice, but must be consistent), and t is the flight time in seconds and and d is bullet diameter in inches:

N=Nm exp[-0.035 t / d]
​

Davers,

Momentum is not a good indicator of stability because it is the product of velocity and mass which don't have the same effect on stability. The gyroscopic stability factor, which takes all the elements of stability into account, increases directly with mass for a given velocity. So, if you keep the same bullet shape and and add enough powder to keep the same velocity, but increase momentum 67% by increasing the mass 67% by replacing the lead alloy core with tungsten, you increase the stability factor 67%. But if you keep the original bullet and increase momentum 67% by adding enough powder to increase velocity 67% you do not get a 67% increase in stability factor. Only about 19%. The reason is that at the higher velocity, air pressure on the bullet nose is greater, so it takes more spin and momentum to keep the bullet from tumbling under those greater aerodynamic forces. That cancels out almost 70% of the gains from the higher momentum and spin.

For example, we take a 168 grain Sierra MatchKing at 2600 fps in a 12" twist barrel. The gyroscopic stability factor is:

s=1.719

We now increase momentum 67% by increasing mass 67% and keeping velocity the same, so the same size bullet now weighs 280.6 grains:

s=2.871 a gain of 1.152

We now increase momentum 70% for the original 168 grain bullet by increasing velocity 67% to 4342 fps:

s=2.040 a gain of only 0.321

So, momentum was increased 67% in both cases, but did not bring about the same effect on stability. It is still bullet length and rate of twist that matter most, which is why the old Greenhill formula only used those two factors.

 

[sionaprhys]

http://www.exteriorballistics.com/ebexplained/index.cfm

Sierra has some good basic discussions of exterior ballistics.

 

[MikeG]

Anybody shoot 1,000 yards? If so, are the bullet holes round or oblong, and if oblong, in what direction?

 

[sionaprhys]

Mine were round.

So were these. http://accurateshooter.wordpress.com/tag/1000-yard/

 

[unclenick]

Yep, they are fairly round, but that really doesn't tell you much. The total elevation on the sights might be something like 42 moa for a .308, so the difference between round (slightly elliptical, actually, as the bullet is dropping in on the down side of an arc and a bullet that entered at the same trajectory tipped up 42 moa (.7 degrees) really ain't much to see clearly. Roughly the difference between a .7 degree ellipse and a 1.4 degree ellipse. They'll both look like round holes.

 

[MikeG]

I figured maybe a .45-70 with a 500 grain bullet would start to make an oblong hole, if the nose didn't follow the trajectory back down.

Anyway.... think I finally have a clear idea in my head why the bullet noses follow the path back down. Bear with me.

Let's say you are using a long pointy cone for a projectile. Instead of launching it point-on, say it's sent downrange with the nose up some ridiculous amount, 45 degrees. And some spin to keep it stable (spin being around the long axis).

Now.... given that there is a lot of cross-sectional area at the base, dropping down to none at the nose.... it might be reasonable to think that the base will have more drag than the nose. And ever so slowly, the base will get pushed behind the nose.

How fast will this happen? Depends on the rate of spin. Just barely enough spin to keep it stable.... the base will get pushed back behind the nose pretty quick as there's little angular momentum to keep it in place. Ramp up the spin to 100,000 rpms (ignore that there could be defects in the cone that might make it not spin true). Result? Lots longer till the drag on the base can overcome the angular momentum and push it back in line. Assume identical forward velocity for both examples.

So.... the higher spin rate on the second cone ends up making it go through the air sideways longer, and ultimately more drag. It slows down faster going downrange and arrives at the target slower.

Ever notice that a beautiful tight spiral on a football heads back to earth point first?

 

[MZ5]

I agree that such a bullet as you describe will 'turn' via air pressure to put the pointy end forward. I realize that I used an 'if' rather than an 'as' in an earlier post, and that was a poor and incorrect choice of word. I've now corrected it.

I don't argue that a bullet fails to have its nose re-directed by air pressure. I argue that the aerodynamic drag change due to how much a bullet noses over, and how much different that nose-over and consequent drag change is in, for example, a 10-twist 308 vs. a 12-twist 308 is veryy small, such that it can effectively be ignored, IMO. And as to the different magnitude of the overall gyroscopic drift resultant from the 2 different twist rates, that's not something to be calculated, it's something to be measured in the field and dialed out with your sighting mechanism.