Bullet stabilizing at distances past the barrel?

[Nytelyte]

Can someone help me understand this? (From the Ruger American .243 at 650 yards... thread)

Of course such a long bullet tends to turn in better MOA results at 200 yards and beyond, potentially not being 100 percent stable at 100 yards.

I can't get through my mind how that could be the case. As I see it, a bullet, which leaves the barrel at a velocity through the air & at a certain spin velocity, both of which will decay as friction through the air imparts its will, will be as stable as it ever can be as soon as it is out of the expansion cloud near the barrel (maximum velocity, maximum spin rate, minimal turbulence).

So how can any bullet become MORE stable as its speed and spin rate slow down? How can it get worse, then get better? How can something that is ever 'unstable' at any range be accurate beyond the point at which it is unstable?

Thanks!

 

[Rainbow Demon]

All long body bullets Yaw when they first leave the muzzle. This causes the nose and tail of the spinning bullet to oscilate.
The longer the bullet the more this affects the bullets flight.
Its also why Armor Piercing Bullets penetrate metal better at two hundred yards than at one hundred yards.

The rotational speed slows little if any as velocity drops.
A pistol bullet fired at an angle into ice can end up bouncing out and landing on its nose then continue spining for minutes while the bullet is at a standstill.

This is why the effect of "Spindrift" increases at longer ranges.

Another effect is "Compensation" of the barrel.
When the barrel is of the best weight and profile for the cartridge used any bullet with less than full velocity will exit the muzzle on the upswing of vibration. Bullets with higher than average velocity will exit on the down swing.
The result is that vertical spread decreases at longer ranges. This effect usually shows up best at between 600-800 yards.

 

[Come and take it.]

So this means a group at 100 yards that measured 1 moa on average, would be less than 1 moa at the longer ranges?

I have a hard time believing that.

 

[jmr40]

This is not common, but not unheard of. You will not see a smaller group size at longer ranges, but smaller MOA is sometimes possible. I've seen rifles that would consistently shoot 1" groups at 100 yards. You would exect that rifle with the same ammo to shoot 2" groups at 200 yards and 3" at 300 yards etc.

Most rifles and ammo combo's will be pretty predictable about this. But I've seen guns would shoot no better than 1"-1.5" at 100, then shoot 1.5" groups at 200 yards and 2" groups at 300 yards.

If you've ever seen anyone spin a top. It will often wobble some at first when it is spinning too fast. After it slows some it will stabalize and will spin perfectly stable until it again slows and once becomes unstable before it falls over. Bullets sometimes do the same thing.

 

[Mike Irwin]

Think of it this way...

Did you ever, as a kid, have a toy top that you would spin with your fingers, drop, and watch it spin?

Remember what it did when you first released it?

It wobbled all over the place, but then it generally settled down into a nice, tight, fairly stable spiral.

A bullet does the same thing.

When the barrel "releases" the bullet (just like your fingers release the top) and it hits the atmosphere (just as when the top hits the ground), it has to make that transition before its spin can fully take over and stabilize it.

 

[Come and take it.]

My interpretation of what is being discussed would be that a bullet would have continually disperse along an outward curve to 100 yards i.e.

It shot at .75 moa (.18-.19 inches) at 25 yards but at 100 yards it grouped at 1 moa (1.1 est. inches). Than once stabilized the shot dispersed at no more than 1 m.oa. (2.2 for 200 yards, 3.3 at 300 yards and so forth) so that afterwards the 1 moa remained a constant along the remaining of the trajectory.

I simply cannot buy into the hypothesis that the bullets once stabilized will by a seemingly conscious decison, bend back toward a common point.

Take for instance your top parable. The top may stabilize to a common center but in that time the tip in contact with the table has wandered. If you spun the top again by the time it has stabilized it will be in a different location on the table and so forth for the third time.

 

[Brian Pfleuger]

They don't bend back toward a predetermined point. The MOA value of the dispersion decreases with distance.

 

[Mike Irwin]

You're misinterpreting it.

The bullet is still going to be directed in a very specific direction based on its travel down the gun barrel.

Also, one has to account for a variety of other factors that make accuracy and group decay non linear, such as imperfections in the bullet and environmental issues external to the gun or bullet.

A better view of the top example are those that are launched with a handle arrangement, not unlike this one: http://www.etsy.com/listing/61189222/spinning-top-handcrafted-wooden

The handle acts almost like the barrel in that it guides the top during its release. It's more stable, but it's still not 100% stable at the point of release.

The top, obviously, differs from a bullet in that the top is the same time and time again while each bullet varies minutely, and the top rides on its nose on a solid surface, which a bullet doesn't do.

 

[Come and take it.]

There are also other factors that can account to an observable decrease in moa at extended ranges.here is one example.

If the gun started out with a clean bore.

a fouling shot was made.

Than groups are fired. The first groups through the cleaner bore may disperse greater than the latter groups because many guns shoot more accurately through a fouled bore than when the bore is clean.

enviromental conditions can also apply.

In a bull barrel that has little warpage as the metal expands from heat so the barrel outwardly and bore inwardly creating a tighter bore which could also create greater accuracy.

An underground testing facility with no enviromental changes and a controlled test making sure that the bore is identically fouled and the barrel temperature is identical each time it is fired would be needed to build on the hypothesis.

Perhaps this research has already been done and someone can post a reference to it, or like any kind of science involving mathematics perhaps there is a mathematical formula that someone could post assuming there is one.

 

[Brian Pfleuger]

It's not a hypothesis. It's a well known fact that Boattail bullets, as an example, are typically less accurate than FB at distances under 3-400 yards but more accurate after.

The "more accurate after" might be obvious but they're not EQUAL at the closer ranges, they're WORSE.

The reason they're worse is that the muzzle blast disturbs the BT more than the FB and it takes it a few hundred yards for the wobble to settle down and it's angle of dispersion decreases to the point that its superior BC out performs the FB even though it didn't at close range.

 

[jmr40]

I simply cannot buy into the hypothesis that the bullets once stabilized will by a seemingly conscious decison, bend back toward a common point.

They don't. You are confusing point of impact with group size.

If a bullet is somewhat unstable at 100 yards, is giving you 1" groups and is perfectly zeroed at 100 yards, conventional wisdom says it will shoot into a 2" group slightly lower at 200 yards. And to be honest this is what happens the vast majority of the time. Bullets rarely become more stable past 100 yards, but it is not unheard of.

But if the bullet becomes more stable past 100 yards, and if every bullet fired in the same group becomes stable at the exact point in flight they may well group into a smaller MOA at greater range. But the point of impact may well be in a different place than expected. The point of impact could well be higher, lower, left, or right of where you expect it to be.

 

[Come and take it.]

I would be more than happy to accept defeat on this,... so long as I get the opportunity to see some data from tank or artillery gunnery or space flight application, demonstrating this behaviour in projectiles.

 

[Brian Pfleuger]

Scaling things dramatically up or down is not always reliable. Evidence of bullets doing things should come from bullets doing things, not from 17,000 mph satellites or 20 pound shells fired miles.

I don't know if this helps you or not but I made the same argument you're making a couple of years ago until some of the wiser (older) guys showed up to set me straight.

It's counterintuitive but, alas, it is fact.

 

[RalphS]

The phenomenon is known as "yaw spiral". Do a google search on "bullet yaw spiral" and you'll find some interesting reading.

There is a good discussion of this on another gun forum by a fellow named "Iowegan". You'll find it in the google search.

 

[Come and take it.]

I am beginning to understand how this is supposed to work.

Perhaps that can help explain some unusual experiences I had with an L1A1 during the mid 90s. I remember shooting 4 inch groups at 100 yards but some time later when a friend of mine had trouble hitting two liter bottles at 300 and 450 yards with his swedish mauser, I was able to bust them with open sights using the l1a1 while he spotted.

The 300 yard 2 liter I busted with the 2nd shot, The one at 450 yards I busted with the first shot.

This could explain what I experienced.


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UPDATE

http://en.wikipedia.org/wiki/Nutation

http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNBLST.html

The phenomenon if I am not mistaken is called Nutation.

Or precession as well.

However I am still puzzled as both precession and nutation still obey my fundamental reasoning in that the oscillations must still obey the center of gravity of the bullet...

 

[Quadpod88]

I absolutely love this thread, what is a good reliable source to do more research on this particular subject?

Pretty please!!

 

[Nytelyte]

There is a lot of very good data here, and info for further search.
Thank you all.

 

[Mike Irwin]

Many years ago this was first phenomon (sp?) was noted very dramatically in some penetration tests done by I believe the US Military.

IIRC Julian Hatcher recounts .30-06 FMJ bullet penetration tests, shooting into pine or some other kind of wood.

At relatively close range the bullets didn't penetrate all that well, and tumbled quickly on hitting the wood.

At longer distances, however, the bullets had far better penetration, and went a lot deeper before they started to yaw.

This was attributed to the fact that at shorter ranges the bullet wasn't yet in its steady state, that it was, as RalphS notes, still in a pretty good yaw spiral.

At longer ranges, however, the yaw spiral had minimized.

Oddly enough, that's a double-edged sword...

With FMJ bullets of that era, most of the wounding was done when the bullets would tumble in flesh. A bullet that tumbles sooner does a lot more damage, so theoretically at least the bullets would be more effective at shorter ranges...

 

[Come and take it.]

Still the bullets are not actually deviating from their path. According to the physics I have read up on. The phenomenon I mentioned both state the "spiral yaw" to revolve around the center of gravity of the bullet.

Or to explain in another way the point and base may change their location and perform a spiral. However the bullets center of mass does not.

You would not see an actual increase in group size followed by a decrease in group size as distance increases (in size by minute of angle).

You may however see what you believe to be a slight keyhole of a bullet at short range which disappears (goes to sleep) as range increases.

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Unless there is still a "spiral yaw" that I haven't found yet to explain an actual spiral path of the bullet.

We see something like this in rockets, but rockets have onboard propulsion and fins. Bullets have neither.

 

[RalphS]

Berger Bullets posted a simulation of what bullet pitch and yaw would look like on a target as the bullet moves downrange and begins to stabilize (go to sleep). http://www.youtube.com/watch?feature=player_embedded&v=4pF8W5liSRc

I still can't visualize this but it does explain why groups shrink as the powder charge is backed off a bit. Slowing down a bullet slows down it's spin rate too and the bullet goes to sleep sooner.