Is it going subsonic that really destroys accuracy?

[DocSkinner]

Thsi si a lead in from another thread - but this aside interested me - hopefully some of those people will follow and comment.

the thread turned into long range ballistics, and .308WINs going subsonic causing instability/inaccuracy and barrel length being the culprit.


I was wondering about whether it is really the going subsonic, or is it more the quality of bullet that makes it start to tumble after so long in flight, and therefore lose great amounts of velocity and fall short.

In handgun silhouette shooting, the bullets start out just above supersonic, but are often very subsonic on the long stages. Why do these bullets not tumble and go scatterwonky, if going subsonic causes that to happen in bullets?

I was wondering about the bullet quality - so in rifles, you get out to the long ranges 500 yd-800yds, and the LONG ranges 800-1000+yds. In benchrest, I know many (all?) people spin their bullets to check for concentricity to varying degrees before loading them, while most other people assume good enough from factory. So now people that don't do that, and those that aren't even using match bullets at long ranges, have to just rely on factory tolerances - which are pretty good as whole. Say out to 500 yds (normal factory bullets) or 800 yds (match grade bullets) before the slight wobble catches up and starts making the bullets unstable and then quickly degrade to tumbling.

This problem would seem to be one that would scale - why .50 cal bullets do well, as it it takes more time and inertia to overcome their ballistic spin, go critical and cause them to tumble.


Or a combination of BOTH transitioning from the supersonic pressure envelope to the subsonic while also having developed a little eccentricity in their spinning causes them to go critical and suddenly start tumbling.

If it is solely the going subsonic, why doesn't it happen to pistol (and other just hypersonic at muzzle) bullets/rounds?

thoughts??

[eaglemike]

Interesting question. I don't think it'straveling at subsonic speeds that causes accuracy problems. This is based on target handguns loads that can be very accurate - and are always subsonic. Just a guess that it would be the transition from supersonic to subsonic that contibutes. Might also be some loss of gyroscopic stability at some point. It'd be interesting to have the time to test this. IIRC the 6.5 X 55 has an excellant reputation for accuracy, and had always been used in fairly fast twist barrels back when the reputation was earned. Not especially high velocity, either. Probaly tests documented somewhere - or ask Randall...

[heycorey]

Could it have something to do with relative length/width of the bullets in question? A long/thin bullet (rifle) would be more susceptible to tumbling than a short/fat bullet (handgun) ... once it's lost its spin.

[DocSkinner]

Could it have something to do with relative length/width of the bullets in question? A long/thin bullet (rifle) would be more susceptible to tumbling than a short/fat bullet (handgun) ... once it's lost its spin.

Pondered about this as well, but seems a basically equal dimension object would tumble more easy, as it doesn't so much matter what axis the bullets spins about?

[ar15barrels]

It's the transistion between supersonic and subsonic flight that upsets the bullet.
Take note that the most accurate 22 rimfire and handgun loads are always subsonic.
Part of this is because they don't transistion.

[rabagley]

As usual, Randall has it right. It's the transition from supersonic to subsonic that causes all of the issues. Once the bullet is subsonic, it moves in a very predictable path again.

Watch the first 30 minutes of "The Right Stuff" again and pretend that your bullet is the Bell X-1 going through that sonic barrier (but slowing instead of speeding up). Supersonic aircraft spend as little time as possible at Mach 1 because you get the crap beat out of you at that speed. You punch through it and get to your cruising speed with all haste.

If you can deal with the rainbow trajectories, you can probably get your tightest groups with subsonic bullets (.300 Whisper and .458 SOCOM both have highly accurate subsonic loads in AR and bolt action).

[Dr Rockso]

Haha, I guess all those aerospace engineering classes are good for something after all. When an object transitions from supersonic to subsonic its aerodynamic center changes. The stability of the object is based on the relationship between the aerodynamic center and the center of gravity, so it's not surprising that it would behave differently at subsonic speeds.

[1lostinspace]

When a rounds transitions from supersonic to subsonic there is turbulence that occurs that will effect a bullets trajectory.

[Pvt. Cowboy]

It's the transistion between supersonic and subsonic flight that upsets the bullet.
Take note that the most accurate 22 rimfire and handgun loads are always subsonic.
Part of this is because they don't transistion.

This is correct. End of thread. Thanks for coming everyone.

[Dr. Peter Venkman]

This is correct. End of thread. Thanks for coming everyone.

I've seen this before, that sniper is just trying to suck us in one at a time.

[DocSkinner]

It's the transistion between supersonic and subsonic flight that upsets the bullet.
Take note that the most accurate 22 rimfire and handgun loads are always subsonic.
Part of this is because they don't transistion.

BUT- 44 mags (et al) transition from supersonic to subsonic at the longer ranges of silhouette -
If you theory is true, why do THEY not lose accuracy in that transition? want new answers - not old dogma...

[ar15barrels]

BUT- 44 mags (et al) transition from supersonic to subsonic at the longer ranges of silhouette -
If you theory is true, why do THEY not lose accuracy in that transition? want new answers - not old dogma...

The longer range targets are a lot bigger.
Ever notice how hard it is to put down those turkeys?
The rams, pigs and chickens are easy...

I started sillouette shooting with 240gr bullets in a 44mag over 23.5gr of H110.
If I were to shoot it today, it would be with 139's in a 7TCU.

[DocSkinner]

As usual, Randall has it right. It's the transition from supersonic to subsonic that causes all of the issues. Once the bullet is subsonic, it moves in a very predictable path again.

Watch the first 30 minutes of "The Right Stuff" again and pretend that your bullet is the Bell X-1 going through that sonic barrier (but slowing instead of speeding up). Supersonic aircraft spend as little time as possible at Mach 1 because you get the crap beat out of you at that speed. You punch through it and get to your cruising speed with all haste.

If you can deal with the rainbow trajectories, you can probably get your tightest groups with subsonic bullets (.300 Whisper and .458 SOCOM both have highly accurate subsonic loads in AR and bolt action).

and I am a licensed A&P that opted out of getting an aerospace engineering degree (and wound up with a BS in psychobio - so top 20% grades in "for majors" chem, bio, and physics, at UCLA, so no slouch). I understand a great deal of the space and Mach challenge era -
the biggest issue not having solid enough control surfaces, and that flexing of those surfaces mislead those engineers for years...

and your argument about spending time close to Mach is faulty - most of our jets spend their most efficient flight at .93 and better mach - so 93% o fteh speed of sound and closer to the speed of sound - the 737s best cruise is at .94Mach - 94% of the speed of sound... Leers gain their fame for flying at .96 to .98 Mach. Modern era is flying at .98 to .99 Mach. Sp steady state at almost teh speed of sound.


a solid entity with little susceptability to outside forces ( i.e.: a bullet) with a highly controlled flight path (surfaces or physics) has little to fear from the transition either way (why your bullet going supersonic in your barrel doesn't destroy your barrel - which according the Bell X1 theories, it shoudl do!), so unless there is some fluctuation or vibratione, or ??? for the pressure changes to act on or to act on the projectile from some slight falw in one direction, there is little reason for the project to tumble when going hyper ot sub sonic. much like how an aircraft or missile or rocket doesn't suddenly tumble when it transitions from hyper sonic to sub sonic.

Laws of physics are highly conserved....
BUT if there were slight irregularities, those irregularities would cause huge problems as they reacted to the change in hyper to sub sonic pressures, and cause a bullet to tumble. while a perfect bullet, prescribing a perfect path, would be affect ed equally in all directions by the pressure change, as all changes would be uniform, no flight alteration would be incurred do to variances.

so if a bullet was perfect, and perfectly launched, it would be concentric, and still spinning concentrically, and so the pressure wave would act the same from ALL directions - which is a null vector - no effect.
but if it was JUST starting to wobble - like the early supersonic attempts that slight flutter wold be catastrophic when amplified by the pressure envelope change.

so that still leaves the question open - if teh bullet was still spinning close enough to perfectly concentric, would the pressure change affect it? and the answer is no -

but a bullet traveling over 900 yards - so 24,300 spins from a 1 in 9 barrel, is likely to develop some wobble or flutter - so any little wobble gets magnified by pressure changes. the pressures at super v sub sonic are huge, and they happen in an instant - so (force/time)*any non zero vector = HUGE deflection! particularly as this is a continuous process - so a slight wobble gets amplified, that larger wobble is amplified o and even greater degree based upon how much more wobble, and you have a run away process that quickly throws bullets, oh say, 15+ minutes of accuracy.

[DocSkinner]

The longer range targets are a lot bigger.
Ever notice how hard it is to put down those turkeys?
The rams, pigs and chickens are easy...

I started sillouette shooting with 240gr bullets in a 44mag over 23.5gr of H110.
If I were to shoot it today, it would be with 139's in a 7TCU.

but are the 44s tumbling at that range? paper targets say no... accuracy sure - iron sights over an 8" barrel... but evidence of tumbling?

[DocSkinner]

The longer range targets are a lot bigger.
Ever notice how hard it is to put down those turkeys?
The rams, pigs and chickens are easy...

I started sillouette shooting with 240gr bullets in a 44mag over 23.5gr of H110.
If I were to shoot it today, it would be with 139's in a 7TCU.

But an 80 degree day at sea level would put your 7TCU rounds at sub sonic before hitting the rams, which means you should not be able to hit rams accurately at 80 degrees at sea level...

Unless the bullets haven't developed enough wobble to too adversely affected by the pressure wave in a a specific direction to cause wobble...

thios would actually be agreat debate to get Barret's, Speers, etc, bullet designers into...