High BC Close Range Bullet Stability

An anecdote just for the heck of it: My pet '06 was reliably just under one MOA for five shots, year after year, at 100 yards. I set up a hanging plate at 500 yards, with the shooting table here by the house. After using up some ammo getting the scope set for 500, I had eight rounds left. I wound up with two four-shot, four-inch groups, or about the same MOA as at 100 yards.
 
Here something from Berger site on VLD bullets as usual Art is right

VLD Bullet Design

The VLD (Very Low Drag) bullet design was born from a request made by the US 300 Meter Shooting Team. It was determined that they were dropping points late in the matches due to recoil fatigue. Bill Davis and Dr. Lou Palmisano were asked to design a bullet and case combination that shot flatter than the 308 case and 168 gr bullets the team was using at the time. After a design was created Walt Berger was approached to make the bullet. The 6mm 105 gr VLD was born and shot by the US 300 Meter Shooting Team using a 2” PPC (modified 220 Swift). This combination shot with less felt recoil and a flatter trajectory than the 308 case using the 168 gr bullet and higher scores were the end result. This successful bullet design soon found its way into all long range target competition and the VLD shape spread into all other calibers.

The VLD bullet design is a combination of two very specific features. The first is a boat tail which is common on long and heavy bullets. The second and most important design feature is the long secant ogive. It is this ogive shape that allows the bullet to experience less drag as it flies to the target. This reduced drag is how the VLD shoots flatter and is less affected by wind (less drift) than other bullets. Reduced drag also translates into higher retained velocity. These are important results if you want your bullet to help improve your accuracy by requiring less sight adjustments when conditions change.
 
In and of itself, systems (like a spinning bullet or spirally thrown footballs in this case) do not typically go from unstable to stable without external influences (like an active control or stabilizing system or fins).
When's the last time you saw a quarterback's 'wobbler' pass become stable on the way to the receiver?
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Even seen a spinning top. They very often go from stable to unstable and back again as their speed slows.

There is debate as to why, but too many experienced shooters have observed this to say it doesn't happen.
 
precession/wobble of the bullet imparted at initial barrel exit that is damped by aerodynamic effects as the bullet travels downrange. The problem with this theory is that the effect would be very minimal.
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The effect being minimal is possibly the best support for the theory. Minimal effects in stabilization would be less apparent at short distances, but as distance increases, the minimal effect is multiplied, therefore more noticable at longer ranges, explaining why the accuracy appears to improve with distance. It is not really that anything is improving, just that it is becoming more apparent.
I haven't taken a physics class since my sophomore year of college, though, so I could just be out of my element here.
 
There are cases of things like 2" @100 yds and 3.5" at 200 yds . More likely it's said with the long bullets of 6.5x55 or 7x57 for example.
I didn't appreciate the problem until I saw a spear throwing video where the spear when thrown wobbled a lot but settled down more and more as the spear went further ! There is a name for this but I never emember it !
 
I have seen this firsthand, a rifle, especially when shootiing long heavy bullets, will shoot 1" at 100, 1.75" at 200, 2.5" at 300, 3" at 400 and so on, it seems that once the bullets have a hundred yards or so to stabilize then they hold true better.
I don't know the physics or science behind it, maybe it was just a freak accident, but i've heard of several other guns that do the same thing. I do however believe that it would be impossible the the physical group size in inches rather than moa get smaller at longer ranges, that would be pure luck.
 
I didn't appreciate the problem until I saw a spear throwing video where the spear when thrown wobbled a lot but settled down more and more as the spear went further ! There is a name for this but I never remember it !
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Porpoising.

I don't think the effect applies to bullets because it involves the arrow/spear actually flexing as it flies.
 
Lots of good comments here on an interesting topic. I agree that a rifle that groups 2" at 100 yards and 1.5" at 200 yards is a physical impossibility. It would in fact be a violation of the Second Law of Thermodynamics since it would indicate an increase in the order of bullet paths as time progressed. Such would only happen in worlds where water runs uphill and perpetual motion machines were possible. On the other hand, many have reliably observed rifles that shoot 1.5" at 100- and maybe 2.0" at 200 yards.

A possible explanation: The axis of a spinning bullet is not stationary; it precesses, which means that the bullet's long axis rotates around the line of the bullets path. Loading manuals mention this in their exterior ballistics sections. Everyone has seen a gyroscope or a top do this. Perhaps the word "yaw" might be used. It may be that the magnitude of the yaw decreases as velocity drops, making the bullet less susceptible to forces causing it to deviate from its intended path. I have no proof of this, nor have I seen any, but I have seen references to a bullet "going to sleep" at range and I think this is what is meant.
 
Art Eatman said:
A bullet cannot speed up in forward motion after leaving the barrel since there is no force on its mass to cause acceleration. The old F = MA.

Similarly, no way its rotational speed can increase. The only force acting on it is the friction of the air--which slows down both rotational and forward speed.
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You are correct in the fact that the spin rpm does not speed up as the bullet flies but the revolutions per foot of travel can go higher because the forward velocity usually decays faster than the spin rpm, and it's the revolutions per foot of travel that determines whether or not the bullet is stable. Otherwise, a .22 rimfire would need a faster twist than a .22 Hornet to stabilize a 40 grain bullet.

As an example, let's consider a bullet shot at 3000 fps out of a barrel with a 1 in 12 rifling pitch. The bullet's initial spin speed is 3000 revolutions per second.
Many yards later, the bullet could concievably slow down to 2000 fps but the spin might have only slowed down to 2500 rps and now it is making one revolution per 9.6 inches of forward travel. It now behaves like a bullet that was shot out of a 9.6 to one barrel at 2000 fps.
 
it's the revolutions per foot of travel that determines whether or not the bullet is stable.
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That's not correct. It's revolutions per time (spin rate--commonly measured as RPM) that makes the difference in terms of bullet stability.

The reason we measure rifling twist in revolutions per distance and get away with it is because for a given caliber the muzzle velocity generally falls within a relatively small range and therefore the bullet RPM at the muzzle is also relatively tightly bounded. The twist, in conjunction with the muzzle velocity is imparting a given RPM to the bullet and that's what's actually important in terms of stabilizing the bullet.

One easy way to see that gyroscopic stabilization is not dependent on spin per distance travelled, but rather on spin rate is to understand that it's possible to stabilize a stationary object (one that's not moving any distance in any direction at all) using gyroscopic effects.
Otherwise, a .22 rimfire would need a faster twist than a .22 Hornet to stabilize a 40 grain bullet.
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It's actually not the weight of the bullet that's important, it's the length of the bullet. Again, it's common to talk about what's needed to stabilize a given weight bullet and we get away with that because for a given caliber and assuming similar construction, similar weights mean similar lengths. That works ok if you're comparing two jacketed spitzers but really breaks down if you're comparing a jacketed spitzer to a lead round-nose bullet, for example. The jacketed spitzer will be much longer for a given weight than the lead round-nose and will therefore require a faster bullet spin for stabilization. That means it would require a faster twist and/or faster muzzle velocity to stabilize than the LRN.
 
That's interesting, I'll have to load some IMR Trail Boss sub-sonic loads in my .22 Hornet to see if a bullet that's stable at normal velocities goes unstable at .22 rimfire velocities.

I have understood that at higher velocities, the upsetting moment caused by the bullet's center of mass being behind the bullet's center of drag is greater and needs more rpm at higher velocities to stabilize the bullet, and at lower velocities, the drag and therefore the upsetting moment is lower and so the spin rpm can be slower.
It's sort of like a top spinning but as the top slows down, the gravity field that tries to tip it over also goes down and becomes zero (weightlessness) as the top slows down to zero. A top spinning in such a scenario would stay upright even when the top was barely spinning because as the spin approached zero, the force trying to tip it over also goes to zero.

I dunno, does a .30 Whisper subsonic round need a faster twist to stabilize the same bullet that a .308 stabilizes with a 1 in 10" twist?
 
I dunno, does a .30 Whisper subsonic round need a faster twist to stabilize the same bullet that a .308 stabilizes with a 1 in 10" twist?
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I believe the most common factory rifling rate for .308 Win is 1 in 12. JD Jones initially recommended 1:8 for the Whisper although some gun companies have gone with slower twists oriented towards use primarily with lighter bullets.

Of course it's not an apples to apples comparison because the Whisper, as originally conceived by Jones, was intended for bullet weights (lengths) that were significantly higher than the typical .308 Win bullet. Still, even so, conventional wisdom has it that 1:10 will stabilize even the heavyweights in .308Win while Jones called for 1:8 for the Whisper.

All that to say that it's clear that Jones believed that the .300 Whisper would require a faster twist than the .308Win.

Also, the table below shows that Berger and Machholz, of Berger Bullets and Sierra Bullets respectively, indicate that a slower twist rate can be used for faster loads in the .300 Whisper. Basically the faster muzzle velocity means that even with the slower twist rate the spin rate will still be high enough for stabilization.

http://feistyrooster.com/300x221/300x221faq.html#barrel
 
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