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Does bullet stability drop of with velocity??

 
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  #1  
Old 06-13-2011, 12:54 AM
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Join Date: May 2011
Location: Auckland, New Zealand
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Does bullet stability drop of with velocity??

Hi there,

I'm developing a hunting load (up to red deer sized game) in my 243 Win with the Swift Scirocco 90 gr (My barrel twist 1 in 10") and I think that I am seeing evidence of "very slight" instability (yawing) by analyzing the holes in my targets. I can't see how I will get good accuracy if the bullet is unstable, so I got on to a bullet stability calculator. The best result from the calculator is that the bullet is unstable/marginally stable (1 in 10", 3250 fps, Projectile Length 1.135"), and I am wondering if it's even worth trying to tweak my load, or if I'm on a hiding to nothing.

Everyone that I speak to says that this projectile should stabilize, but the maths/science says to me that I'm wasting my time?

I guess what I'm asking is this:

1. Am I wasting my time?
2. Will the bullet become even less stable in flight (as the velocity drops off) or not.
3. Will I be better off using a 85gr Nosler Partition? The stability calculator claims that this bullet is better suited to my set up.

Cheers
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  #2  
Old 06-13-2011, 02:30 AM
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Join Date: Apr 2009
Location: Carrollton, Ohio
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Re: Does bullet stability drop of with velocity??

Quote:
Everyone that I speak to says that this projectile should stabilize
I get that a lot; but have found the calculators to be correct.

Quote:
I can't see how I will get good accuracy if the bullet is unstable,
Using loads I've developed with bullets I can't stabilize, I get 1/4" larger groups at 400 yards, than with bullets I can stabilize. I don't see it affecting accuracy, but energy at extended ranges.

Quote:
3. Will I be better off using a 85gr Nosler Partition? The stability calculator claims that this bullet is better suited to my set up.
I use the longest bullet the calculator suggests.
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Old 06-13-2011, 02:40 AM
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Join Date: May 2011
Location: Auckland, New Zealand
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Re: Does bullet stability drop of with velocity??

Thanks for that,

So firstly, do you think that the projectile will become more and more unstable as it flies, or do you think that it won't be an issue.

I have to say that I really really like the Scirocco, but I just don't want to waste my time / money chasing a dream that will never happen.

I'm getting very good velocities from these projectiles.

I almost think that I should try another ladder test and see if they come right. The one thing that I have learnt since my first ladder test is that they should be loaded 50 thousandths of an inch off the lands, and my first test was 20 thousandths off.

I have read that that makes a huge difference.

I will say that the yawing is not extremely evident, but I do believe that the projectile is yawing at 100m.

I would appreciate your thoughts.
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  #4  
Old 06-13-2011, 09:29 AM
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Join Date: Apr 2010
Location: Allen, TX
Posts: 2,608
Re: Does bullet stability drop of with velocity??

My understanding is that once the bullet reaches gyroscopic stability, it stays that way pretty much until impact except for the transonic range which is really a separate issue. If it's unstable to begin with it probably won't get much less so. But, it's going to seem like it over long distances where you need better accuracy for a clean kill.

Depending on your bullet, then your impact velocity should ideally be above 1800 fps for good terminal performance on game which is well above transonic. ...perhaps 2000fps or more for the Scirrocco?

Gyroscopic stability is one factor affecting accuracy. You can sometimes find a load that matches the harmonics of your rifle and it will provide sufficient accuracy with a marginally stabilized bullet. You would just have to try it.

I've wasted a lot of time trying to make slow twist rifles shoot well with longer bullets. Life is too short. I prefer to go by the calculator and focus on one that's a mathematical good fit.

-- richard
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  #5  
Old 06-13-2011, 10:40 AM
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Join Date: Nov 2005
Posts: 1,081
Re: Does bullet stability drop of with velocity??

I may be wrong, but you may be on the very low edge of stability.

Now that means if your crown is perfect, the gas exits perfectly square and the temperature is high and barometer is low then you should be fine

On the other hand, a marginally stable bullet can't recover from yaw at the muzzle and in high pressure or low temperatures the bullet it may not ever be stable even under ideal conditions.

edge.
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  #6  
Old 06-13-2011, 11:27 AM
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Join Date: Feb 2010
Location: Gillette, WY
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Re: Does bullet stability drop of with velocity??

Quote:
Originally Posted by rscott5028 View Post
My understanding is that once the bullet reaches gyroscopic stability, it stays that way pretty much until impact except for the transonic range which is really a separate issue. If it's unstable to begin with it probably won't get much less so. But, it's going to seem like it over long distances where you need better accuracy for a clean kill.


-- richard

^^^This is how i've come to understand it as well, rotational velocity does not bleed off like translational velocity, your round should retain the majority of its rotational speed throughout the bullet flight.

But here is the Caveat, if your are experiencing bullet yaw on paper it's no big deal, but you throw that same amount of yaw on a game animal you may be setting yourself up for a serious bullet failure. As bullets are designed to obviously penetrate in as straight a line as possible, gyroscopic stability is a must, if your bullet either hits the animal with a slight yaw or yaws while in the animal you will tend to see unpredictable penetration. Your shoulder shot may end up somewhere in the ham etc (think of it as a deflection). Also at work are the mechanical forces on the bullet itself, being designed to impact at the nose, they don't tend to hold up with all of that force being applied to side of the bullet.

Just my .02.
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  #7  
Old 06-13-2011, 03:05 PM
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Join Date: May 2011
Location: Auckland, New Zealand
Posts: 82
Re: Does bullet stability drop of with velocity??

OK, well from all the posts that I've received, it seems that:

1. The problem is mathematical. ie Trust the stability calculator more than the guy with one twitching eye in the gun shop.
2. The stability calculator says that the stability is only 0.87 and therefore not stable.
3. The projectile is unlikely to become more or less stable in flight.
4. I can't change any practical parameters to the extent required to stabilize this bullet out of this barrel.
5. Terminal performance would be unpredictable if the projectile was still yawing on impact.

Therefore, sadly, my conclusion is to give up on the Sciroccos and try something shorter.

Would you guys agree with my logic and my outcome here??
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