Bullet Stabilization

[TXAZ]

I read JohnKSA post on spin:

http://thefiringline.com/forums/showthread.php?t=530927
I know the caliber, bullet mass, velocity and other parameters come into play.

Are any of you aware of any good resources that provide a comprehensive discussion on optimizing spin for any projectile? Equations, theories or other relevant information would be appreciated.

Thanks

 

[Jim Watson]

Wikipedia has a discussion of the Miller Stability Formula with references back to the long form of the old Greenhill equation and links to other calculations.
https://en.wikipedia.org/wiki/Miller_twist_rule

JBM will figure it for you.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

 

[JohnKSa]

Here's some information about a person who came up with a very simple formula for calculating rifling twist rate to insure stabilization in certain bullet types. The writeup includes information on his formula.
https://en.wikipedia.org/wiki/Alfred_George_Greenhill

This link contains a calculator that uses a different formula and an explanation of how it differs from Greenhill's formula and why.
http://kwk.us/twist.html

Here's another calculator based on yet another formula.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
And an explanation of the formula used.
https://en.wikipedia.org/wiki/Miller_twist_rule

Some interesting reading on twist rates and stability.
http://bisonballistics.com/articles/barrel-twist-and-bullet-stability

Part 1: http://arxiv.org/ftp/arxiv/papers/1410/1410.5340.pdf
Part 2: http://arxiv.org/ftp/arxiv/papers/1410/1410.5737.pdf

http://arxiv.org/ftp/arxiv/papers/1401/1401.4187.pdf

 

[Llama Bob]

This is all good information, but it's worth noting that for most practical uses twist rate shouldn't be "optimized". It should be overdone. If you have too little twist rate, your bullet is unstable with the resulting loss of accuracy, ballistic coefficient, and in the most extreme cases keyholing.

In contrast if your twist rate is excessively high, generally nothing whatsoever bad happens. With modern bullets which have very uniform densities and jacket thicknesses, there is essentially no negative accuracy impact. It is theoretically possible for a lightly constructed bullet to spin apart, but outside of a few hyper velocity cartridges this doesn't happen.

The trend over the last few years is towards longer bullets (boat tails, VLD, voids, polymer tips, voids, etc.) and copper bullets. Both require more a faster twist rate than is traditional.

 

[Jim Watson]

Benchrest shooters tend toward minimum twists.
Things like 14 twist 6mms. But they are only shooting 68 grain bullets.
Likewise 13 twist .30s for 132 grain bullets.

 

[Colorado Redneck]

A potential unwanted effect of "overspin" is projectiles that disintegrate or overheat due to fast spin. I have a 9 twist 22-250 and some of the more popular varmint bullets will not work. For example, 40 gr. V-Max don't work at 22-250 velocities. However, 53 gr. V-Max work great. Nosler markets a 40 gr. .224 diameter bullet that is non-toxic and advertised to work with fast twist rifles.

 

[bedlamite]

A potential unwanted effect of "overspin" is projectiles that disintegrate or overheat due to fast spin. I have a 9 twist 22-250 and some of the more popular varmint bullets will not work. For example, 40 gr. V-Max don't work at 22-250 velocities. However, 53 gr. V-Max work great. Nosler markets a 40 gr. .224 diameter bullet that is non-toxic and advertised to work with fast twist rifles.

A few years ago I read a post from Eric Stecker from Berger on another forum. They did some testing where they took bullets to the failure point, and their conclusion was that the primary cause of bullet disintegration was heat from barrel friction, and a faster velocity produced more heat. If the bullet core reached melting point, the jacket could not contain it, even with a slow twist. As long as the core stayed solid it would stay in one piece and could be shot much faster, regardless of twist. They came out with the thicker jackets for their target bullets because of this, It can absorb more heat and takes longer for the heat to conduct to the core.

 

[Colorado Redneck]

Before buying the 9 twist barrel, research about fast twist lead me to some threads on various forums (one or two were in TFL) about rpm calculations and bullet integrity at fast rpms. So, I contacted Hornady about what V-Max bullets would work for me, and was told by their tech that 40 gr. V-Max would come apart over a certain rpm (can't recall and at this point don't want to chase down the e-mail ) but the 50 V-Max and 53 V-Max were fine. Then contacted Sierra tech and asked about their Blitzking bullets. Their answer was that all of their Blitking bullets are OK up to---IIRC---340,000 rpm. I had some 40 gr. Blitkings and they work fine, but in this barrel I like the 53 V-Max best.

This from AccurateShooter.com
http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

However, if you ask a typical reloader for the rotational rate of his bullet, in revolutions per minute (RPM), chances are he can’t give you an answer. Knowing the true spin rate or RPM of your bullets is very important. First, spin rate, or RPM, will dramatically affect the performance of a bullet on a game animal. Ask any varminter and he’ll tell you that ultra-high RPM produces more dramatic hits with more “varmint hang time”. Second, RPM is important for bullet integrity. If you spin your bullets too fast, this heats up the jackets and also increases the centrifugal force acting on the jacket, pulling it outward. The combination of heat, friction, and centrifugal force can cause jacket failure and bullet “blow-ups” if you spin your bullets too fast.

 

[JohnKSa]

With modern bullets which have very uniform densities and jacket thicknesses, there is essentially no negative accuracy impact. It is theoretically possible for a lightly constructed bullet to spin apart, but outside of a few hyper velocity cartridges this doesn't happen.

"Overstabilizing" a bullet can have a detrimental effect when shooting at long ranges. Basically what happens is that the bullet is so stable that it aerodynamic forces can't reorient the bullet to keep the nose pointing in the direction of travel as it traverses the arc to the target.

Normally, a bullet would start out nose up, level out at some point and then go nose down as it descends downward to the target. Obviously a bullet pointing nose forward is more aerodynamic. If it is overstabilized, the nose remains pointing upwards throughout the entire flight which means that as it descends, it is less aerodynamic than it should be. It's not a huge effect, and it would be negligible at close ranges.

Benchrest shooters tend toward minimum twists.

And that's what the bullet manufacturers will tell you too. Don't spin the bullet faster than you need to. That's not to say that there shouldn't be enough of a margin so that on a cold day the reduced muzzle velocity causes the bullets to become unstable, but it does mean that using a twist that's 40% faster than it really needs to be isn't helping you any.

 

[Llama Bob]

"Overstabilizing" a bullet can have a detrimental effect when shooting at long ranges. Basically what happens is that the bullet is so stable that it aerodynamic forces can't reorient the bullet to keep the nose pointing in the direction of travel as it traverses the arc to the target.

Is there any evidence whatsoever that this happens in small arms (not artillery)? Because I've shot rifles that are roughly 2x over the required twist with very loopy trajectories - .45-110s shooting 400 grain nitro loads with a 1 in 18" twist for example - and I've seen no evidence of projectiles impacting anything other than nose first, or seen any reduction in BC like you would expect if there was a problem. They seem to do what the ballistic calculator says they'll do out to 1000y or so.

I'm inclined to say this is a myth unless there are specific conditions that are known to reproduce it.

 

[JohnKSa]

Artillery and small arms differ only in scale. The fact that it happens in artillery means it can also happen in small arms although the magnitude of the effect might reasonably be expected to be significantly different due to the significant differences in scale.

Stating that your experimentation hasn't produced the effect isn't sufficient to brand it a myth unless you have determined that the tests you performed and the accuracy of your measurements is such that they would detect the expected effect.

I haven't worked out the math to determine at what point overstabilization would occur in small arms, nor how much it would affect the ballistic coefficient for a given amount of "excess spin". It sounds like a fun exercise for someone who has a lot of ego wrapped up in proving it does or doesn't exist. (i.e. not me.)

 

[Llama Bob]

I also found it interesting that even the most arcing rifle trajectory is actually pretty flat. I just ran the math on that .45-110 load (Speer 400gr at 1700 ft/s), and surprisingly it's only traveling at a 6.6 degree down angle (relative to absolute horizontal) when it hits the buffalo target at 1000 yards.

Maybe the trajectories are just too flat for it to matter.

 

[Llama Bob]

The reason I bring up the example is that a gun like a medium speed .45 caliber with a stability factor of over 6 at sealevel/0deg F seems like about the most extreme example I can think of. It's roughly analogous to a 1 in 5.5" twist .30 cal, for example. If nothing weird happens there, I can't imagine what rifle/load combination would be required to make it happen.

 

[JohnKSa]

...surprisingly it's only traveling at a 6.6 degree down angle when it hits the buffalo target at 1000 yards...

As a very rough back of the envelope, that would suggest a 10-12 degree misalignment from the ideal might be possible by the time the bullet impacts since it would have started out with a significant up angle when initially fired.

This guy claims to have seen the effect.
http://bulletin.accurateshooter.com...-spin-a-problem/comment-page-1/#comment-42710

"I’ve seen it myself pushing 6.5 mm 85 gr pills through a 1:7.25 barrel, where identically oriented keyholes appeared on the targets at 500 meters and on."​

I wonder if there's something else at work there as I'm not sure that the expected misalignment would be sufficient to be easily detected on target.

Some interesting reading which includes the topic at hand.
http://www.nennstiel-ruprecht.de/bullfly/

 

[Jim Watson]

I just ran the math on that .45-110 load (Speer 400gr at 1700 ft/s), and surprisingly it's only traveling at a 6.6 degree down angle (relative to absolute horizontal) when it hits the buffalo target at 1000 yards.

The late Dan Theodore ran the numbers and did actual range tests with spaced targets in the 1000 yard butts. Real Creedmoor ammo with say a 520 at 1250 was coming in around 4.5 degrees.
So much for the "mortar like trajectories" of the old rounds.

 

[impalacustom]

Go have a read at Bryan Litz's book, it'll give you all the formulas and information you could want to know.
http://appliedballisticsllc.com/

Over stabilizing a bullet at extreme ranges will result in a yaw effect. I have blown up a few bullets before by overspining them as well, nice grey puff of smoke about 30ft in front of you.

 

[stubbicatt]

I am far from an expert on this subject. I have recently taken up schutezen rifle shooting, with breach seated, cast, plain base bullets. There is a lot more to it than I know, but when you examine the 200 yard targets, you sometimes notice nose high strikes, which is an indication to back off your powder charge to reduce rpm. Like every shooting sport higher velocity tends to get you a flatter trajectory and a shorter time of flight to minimize wind drift, so most fellas try to find a twist rate to not overspin the bullets, but minimize flight time. -Of course there is an effective maximum velocity that one can attain due to the plain based lead bullets, without leading.

I have learned that between about 800 and 1800 fps or so is a very unstable velocity zone for most bullets, and they are supposedly the most affected by wind at these speeds. Unfortunately that is the velocity zone for the old black powder cartridges. In this zone the short nose "Pope" style bullets seem to perform the best. These bullets are characterized by really short flat noses, and the ones I shoot are tapered to fit the throat of my chamber better. So bullet shape, weight, length all these things need to be optimized for the shooting one intends to do in a rifle of a given twist.

Though recently a lot of fellas are shooting a sort of spitzer bullet. It is interesting to be on the firing line and listen to the really detailed explanations of these things, most of which is beyond my reach.

Seems the happiest place for the bullet to be is *just* stabilized, as in this way the bullet shape follows the arc of trajectory.

 

[Llama Bob]

As a very rough back of the envelope, that would suggest a 10-12 degree misalignment from the ideal might be possible by the time the bullet impacts since it would have started out with a significant up angle when initially fired.

Sorry - I didn't explain myself very well at all. That number assumed a flat trajectory to start and asks what the angle would be by 1000y. Obviously that's not a 1000y zero.

If you zero at 1000y, I get you start with an up angle of 2.5 degrees, and end up going 4.3 degrees downwards by the time you hit the target which matches Jim Watson's number very well considering mine was a nitro load.

"I’ve seen it myself pushing 6.5 mm 85 gr pills through a 1:7.25 barrel, where identically oriented keyholes appeared on the targets at 500 meters and on."​

I wonder if there's something else at work there as I'm not sure that the expected misalignment would be sufficient to be easily detected on target.

I'm pretty sure there's something else going on with that report. Even if the bullet was misaligned 7 degrees (which I think is a reasonable max for very high trajectories) I don't think that's visible in the target. A bottleneck 6.5 is going to be a lot flatter, especially at 500.

This also gets at the practical difference between artillery and small arms. While the physics is the same, you may be trying to get an artillery shell to precess through 90+ degrees, not 7.

 

[Brian Pfleuger]

Ask any varminter and he’ll tell you that ultra-high RPM produces more dramatic hits with more “varmint hang time”.

And any varminter that told you that lacks a good understanding of the principle of Conservation of (Angular) Momentum and physics in general.

The force imparted by the spinning of the bullet is effectively zero.

The numbers SOUND impressive. A bullet can be spinning 300,000rpm. Sounds pretty amazing. However, if that bullet came from a 1:9 barrel, it has to travel 9 inches to make one revolution (at the moment it leaves the muzzle). A varmint is, what, 6 inches thick if it's a big one? The bullet does 2/3rds of a revolution while it's in there. If it's slowed down to 2,000fps on impact and the RPM hasn't changed at all (it won't change nearly as much as velocity but does a little) the bullet does 1 revolution inside the critter.

Plus, that bullet weighs maybe 50gr? Even if it's 100gr, that's less than 1/4 of an ounce. Even a really light varmint weighs 40 or 50 times as much. Even a small (1lb) adult prairie dog outweighs that 100gr bullet by 75 times. A big one weighs 200 times as much or more.

PLUS that momentum only has a lever equal to the radius of the bullet with which to impart force (torque). That'd be (less than) 1/8th inch if it's a .243.

In other words, bullet rpm is irrelevant to the effect on target.

 

[bedlamite]

In other words, bullet rpm is irrelevant to the effect on target.

The only effect I could see bullet RPM having is that centrifugal force helps open the bullet faster. Otherwise you're right, it's velocity doing the work.