High BC Close Range Bullet Stability

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mwsenoj

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I have heard on the forums and recently from a buddy how some of the long high bc bullets that are very accurate out at medium to long range may not stabilize and show up in proportionately small groups at closer range (100 yds). I am having a hard time wrapping my mind around this idea that a bullet's accuracy increases with range. My common sense tells me to call BS, do any of you have any more info on this?
 
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The Sierra 52-grain HPBT is relatively long, although not as long as the rather-heavy bullets. I was getting reliable 3/8 MOA from my Swift, on my 100-yard range.

I hear all this stuff about down-range stabilizing, but I've seen way too many sub-MOA groups to believe it.
 
I would have to say the science is wrong on this one.

When the bullet leaves the barrel, it will have the max rotation imparted on it right away. It will not gain anymore rotation as it travels long range but actually slow down due to drag and gravity.

Also if the bullet was not stable to start out with coming out of the barrel, it would more likely to become less stable due to the effect of drag. For it to become more stable, it would have to gain more rotational speed as it travels in distance. But from my point above, this is highly unlikely.

But then again, I could be wrong :)
 
Some military test have shown that bullets can increase in speed up to 80 or so meters after leaving the barrel. I'm not sure how accurate this is, it was a small blurb in a study on something entirely different. I think they'd set up chronos along the bullets flight path every 10m... But I don't remember exactly. If I can find it again I'll post it.

I heard the same thing about .50 cal bullets not stabalizing until 500m. Didn't believe it then, still don't... But that was from my section leader almost a decade ago.
 
I think a common sense approach debunks this. Assuming the premise is true, after becoming stable, the bullets would still continue on the now altered and imprecise/inaccurate trajectory. Inertia doesn't suddenly disappear. even if the bullet becomes stable somewhere down range and no longer has errors in accuracy and precision being induced do to the instability. The stabilized bullet would still be on an imprecise/inaccurate trajectory, which would be further compounded by the other external forces that are acting on it.

Unless you're shooting some of those experimental guided bullets, the odds of making a good group after stabilizing are probably about the same as making a good group at long range with a smoothbore musket...not likely.
 
Rotational stability is not a simple concept. Objects rotating at high speeds can do some weird stuff... We've all seen the epicyclic yaw/pitch motion of a bullet due to procession?
http://www.youtube.com/watch?v=KH9SCbCBHaY
You see a little bit of a wiggle in flight path (right graph) there at the very beginning (sub 100yards). It wouldn't be noticeable on paper, but it is there...

Here is an interesting study that was done. A nonlinear 6-DOF model of a .50 cal bullet being launched at different sideways angles from a subsonic aircraft.
http://www.ima.org.uk/_db/_documents/Gkritzapis.pdf
Granted, this isn't exactly like sitting at a nice quite range, but on the last page you can see clearly that oscillating velocities perpendicular to the bullets flight path were present, and which were damped out over time (at roughly 450meters).

Could there then be a scientific explanation for this phenomenon? I think so...
If you get enough of an initial yaw angle rate, whether via shooting out the side of a moving aircraft or with too long/skinny a barrel that whips, instability at short range seems feasible...
 
I don't think it's so much as a high BC BT type bullet being more accurate at a long distance compared to a shorter distance, but a higher BC bullet will hold a higher velocity and more energy and be less susceptible to wind drift at longer distances so in those regards it could be more accurate as say a flat based lower BC bullet.
 
I don't think the bullets BC has anything to do with it. It seems to be a rare thing among some rifles, not ammo or bullets. It is not common really, but I have seen rifles that would only shoot around 1.5" at 100 yards. You would expect that gun and load to shoot around 3"" groups at 200 yards, but I've seen guns that would only do 1.5" at 100 yards and still keep all shots under 2" at 200 yards and continue to hold 1 MOA out to longer ranges.

I have not seen,or heard of a gun actually get better at longer ranges. For example I've never seen or heard of a rifle shooting 2" groups at 100 yards and getting 1.5" 200 yard groups.
 
"I learned a lot about epicyclic swerve from this project. The bottom line is that epicyclic swerve cannot cause smaller angular groups at longer ranges. " Author of the Simulation

It seems there's a partial truth here. Bullets become more stable(not unstable to stable), however this increased stability has no appreciable effect on short or long range accuracy.

I'd guess that there is some other mechanism at work if people really are shooting lower MOA groups at longer distances.

Mr Litz does seem to have the credentials to do a reasonable simulation assuming this is true.
 
Marquezj16 said:
Marquezj16
I would have to say the science is wrong on this one.

When the bullet leaves the barrel, it will have the max rotation imparted on it right away. It will not gain anymore rotation as it travels long range but actually slow down due to drag and gravity.

Also if the bullet was not stable to start out with coming out of the barrel, it would more likely to become less stable due to the effect of drag. For it to become more stable, it would have to gain more rotational speed as it travels in distance. But from my point above, this is highly unlikely.

But then again, I could be wrong
Click to expand...

You're right, and Sir Issac Newton's first law agrees with you.

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?

On the flip side,a kid throwing a yard dart may seem like a different issue, as is a flechette shot out of a gun backwards. But it's really the same It will likely achieve stability because the least drag attitude happens to be most inherently stable, and the external forces on both of these examples apply the maximum forces when the projectiles are not in their lowest drag configurations. When it's perfectly pointed into the wind stream, the external forces are minimal.

If there is any 'stabilizing force' right out of the barrel, it has to be minuscule compared to the inertial forces of the bullet. I don't see bullets becoming more stable.
 
have experienced this first hand with rem 22 mag v-max loads and a cz rifle. Groups were so so at 50 yds, outright amazing at 100 yards, consistently.

Not all bullets behave the same.
 
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.
 
What about this theory?

A bullet is overspun by the barrel and is travelling too fast at the first 100 yards, but beyond that the spin has slowed down and its not moving as fast. So the bullet is more stable and therefore has less deviation in flight after it has decelerated.

Possible?
 
...beyond that the spin has slowed down...
Click to expand...
The information I've seen indicates that rotational velocity decreases very little as a result of air friction. Makes sense.

The best theory I've seen to try to explain the "bullet going to sleep" effect that supposedly results in smaller groups at long range than close range has to do with 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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