Keyholing at 1000 yards

[Charles E]

Is this always the case?

Pretty much. You could probably construct a scenario where it wasn't true, but it would be an unlikely one.

The thing for everyone to take away from all this is that rate of twist is not the only thing that affects stability. If you are having troble. consult a *good* ballistician -- read a book. And not Wham Bam Good 'Ol Wives Tails [sic Tales?]

 

[4Mesh]

Moreover, Sg tends to increase as a bullet files downrange, regardless of the velocity.

Pretty much. You could probably construct a scenario where it wasn't true, but it would be an unlikely one.

Would it be just semantics to say that the transonic region destabilizing a bullet is not a reduction in Sd? At that point where a bullet tumbles, is has lost virtually all Sd has it not? Ie: Destabilized, both G'ly/D'ly?

 

[Charles E]

Would it be just semantics to say that the transonic region destabilizing a bullet is not a reduction in Sd? At that point where a bullet tumbles, is has lost virtually all Sd has it not? Ie: Destabilized, both G'ly/D'ly?

Well, a tumbling bullet is certainly not stable. It isn't so much a matter of semantics as causes. There is empirical data, it can be correlated with physical properties -- the whole point of modeling, of theories. So, for example, when you take that .308 Winchester and the 168 SMK tumbles at 1,000 yards, one thought might be to go to a faster twist. But it doesn't work. Wasted money. But wait! The model predicted that, so you don't have to waste your time & money. I wouldn't call that semantics.


BTW, not all bullets tumble when passing through the transonic region. That is where the current model lets us down a bit. AFAIK, we can't yet know in advance just which bullet designs will be "dynamically unstable"; though the smart money is on the boattail design.

 

[ewallace]

this is a target I shot in a Iowa 1000 yard BR match 6.5x284 8.5 twist prototype bullets these bullets shot very good at 900 yards at my home range but not so good in the competition

 

[nhkuehl]

Thanks for posting the picture. Do you know what your velocity and Sg was? What was the bullet weight and length? - nhk

 

[4Mesh]

I have experienced the same thing as EWallace using a 210 Berger VLD in a 12 twist. At 30.5" barrel length the bullets shot fine. At 29", they did as above. The only difference being, I did it in practice.

 

[Mark IV]

If you call the balistics engineers at Sierra, they will tell you that the 16 grain MK fired in a .308 goes subsonic as it approaches 1,000 yards and becomes unstable. The result is that the bullet will key hole. They make the 175 grain MK to fix this problem because it stays supersonic past 1,000 yards.

Sierra has some really knowledgable engineers who are glad to answer calls over the phone.

 

[Mark IV]

If you call the balistics engineers at Sierra, they will tell you that the 168 grain MK fired in a .308 goes subsonic as it approaches 1,000 yards and becomes unstable. The result is that the bullet will key hole. They make the 175 grain MK to fix this problem because it stays supersonic past 1,000 yards.

Sierra has some really knowledgable engineers who are glad to answer calls over the phone.

 

[R.G. Robinett]

If you call the balistics engineers at Sierra, they will tell you that the 16 grain MK fired in a .308 goes subsonic as it approaches 1,000 yards and becomes unstable. The result is that the bullet will key hole. They make the 175 grain MK to fix this problem because it stays supersonic past 1,000 yards.

Sierra has some really knowledgable engineers who are glad to answer calls over the phone.

What they should have told you: a bullet which becomes 'unstable', as it transitions from super, to sub-sonic, is a dynamically unstable design: see Charles E's posts above.
Regardless of the RPM/Sg, if the Sd isn't there, neither velocity, nor additional spin-rate will help: some bullet designs just don't work. IF the bullet passes through the 'transsonic' region, and comes out stable, it IS a dynamically stable design: staying super-sonic is not, 'where it's at'. RG

 

[Lynn]

Here is a little bit of trivia I saved from the lost river bullet debates held on anoother forum from many years ago.There used to be a french poster here Eric Danis from the french company dantec who would show up at shot show who knew this stuff forwards and backwards.

The Balanced Flight/Controlled Spin Projectile bullet patent has been questioned/disputed by the German physicist Lutz Möller.[8] Mr. Möller realized balanced flight has to do with the nature of spin stabilized flight and scale of parameters. There were projectiles produced prior to the patent that remain stable through the transonic flight regime. This is a consequence of the spin deceleration and forward deceleration of the projectile being similar enough not to cause undesirable precession and yaw during the transonic flight phase. The main parameter for achieving stable transonic transition is controlling the drag coefficients (Cd) and forward velocity loss around Mach 1 and to a lesser degree controlling the spin deceleration.

If you do a search for Dean Michaelis doppler radar transonic region testing you will get a wealth of photos showing the actual bullet in flight as it enters the transonic region.Testing from the Yuma proving grounds in those days was around $50,000.I sent them first hand to Henry Childs many years ago but no longer have them on my home computer.
It was definitely the hot topic back then.
Lynn

 

[Charles E]

Regardless of the RPM/Sg, if the Sd isn't there, neither velocity, nor additional spin-rate will help

Actually, velocity will help -- if the extra velocity keeps the bullet above the transonic region at the target. The rest of R.G.'s post is spot on.

With regards to Lynn's post. I have a glimmer that this testing was done to promote a certain bullet, rather than to test projectiles generally. I could well be wrong.

It is easy, but expensive, to get empirical data on the Sd of existing bullets. Bob McCoy, using the facilities at Aberdeen, was also able to get empirical data. What we don't have, to the best of my knowledge, is a model that accurately *predicts* the Sd of a bullet. If we had that, we'd know what design parameters to avoid.

Practically speaking, avoiding the transonic region is one simple method. I've often said, "whatever happens there, it isn't good" -- it may not be bad for some bullets, but it's sure no sweet spot.

If, for some reason, you must shoot a chambering where staying above the transonic region just isn't possible, you're in for some testing, or getting bullet recommendations from those who've been there -- like guys who shoot a .308 at, say, 1,500 yards.

 

[Lynn]

Charles E
You are right as the big debate back then was about the then new 408 chey-tac cartridge and its monolithic solid bullets and the balanced flight patent.Most of the data is still on the wikipedia external ballistics page.

The transonic problem
When the velocity of a rifle bullet fired at supersonic muzzle velocity approaches the speed of sound it enters the transonic region (about Mach 1.2–0.8). In the transonic region, the centre of pressure (CP) of most bullets shifts forward as the bullet decelerates. That CP shift affects the (dynamic) stability of the bullet. If the bullet is not well stabilized, it can not remain pointing forward through the transonic region (the bullets starts to exhibit an unwanted precession or coning motion that, if not dampened out, can eventually end in uncontrollable tumbling along the length axis). However, even if the bullet has sufficient stability (static and dynamic) to be able to fly through the transonic region and stays pointing forward, it is still affected. The erratic and sudden CP shift and (temporary) decrease of dynamic stability can cause significant dispersion (and hence significant accuracy decay), even if the bullet's flight becomes well behaved again when it enters the subsonic region. This makes accurately predicting the ballistic behaviour of bullets in the transonic region very difficult. Further the ambient air density has a significant effect on dynamic stability during transonic transition. Though the ambient air density is a variable environmental factor, adverse transonic transition effects can be negated better by bullets traveling through less dense air, than when traveling through denser air. Because of this marksmen normally restrict themselves to engaging targets within the supersonic range of the bullet used. [12

Doppler radar measurement method
Governments, professional ballisticians, defence forces and a few ammunition manufacturers can use Doppler radars to obtain precise real world data regarding the flight behaviour of the specific projectiles of their interest and thereupon compare the gathered real world data against the predictions calculated by ballistic computer programs. The normal shooting or aerodynamics enthusiast, however, has no access to such expensive professional measurement devices. Authorities and projectile manufacturers are generally reluctant to share the results of Doppler radar tests and the test derived drag coefficients (Cd) of projectiles with the general public.

In January 2009 the Finnish ammunition manufacturer Lapua published Doppler radar test-derived drag coefficient data for most of their rifle projectiles.[13][14] With this Cd data engineers can create algorithms that utilize both known mathematical ballistic models as well as test specific, tabular data in unison. When used by predicative software like QuickTARGET Unlimited, Lapua Edition[15] this data can be used for more accurate external ballistic predictions.

Some of the Lapua-provided drag coefficient data shows drastic increases in the measured drag around or below the Mach 1 flight velocity region. This behaviour was observed for most of the measured small calibre bullets, and not so much for the larger calibre bullets. This implies some (mostly smaller calibre) rifle bullets exhibited coning and/or tumbling in the transonic/subsonic flight velocity regime. The information regarding unfavourable transonic/subsonic flight behaviour for some of the tested projectiles is important. This is a limiting factor for extended range shooting use, because the effects of coning and tumbling are not easily predictable and potentially catastrophic for the best ballistic prediction models and software.

It still makes sense just to avoid the transonic altogether.
Lynn

 

[alinwa]

Good answers to a Good Question

al