Over stabilization

[Twud]

Is it possibe to over stabilize a bullet. How would you know?
I'm stuck between 1/12 and 1/10 for a new 6 BR. The gun will be a varminter, but it would be cool to be able to shoot some heavier bullets for deer. 65 to 70 grain for hogs 85 grain for deer. Would a 1/10 twist overstabilize a 65 grain varmint bullet? Would a 1/12 twist under stabilize an 85 grain Nosler Partition?

 

[USAPatriot]

I don't know that it's possible to overstabilize, no. Ideally the bullet is going to go down range rotating on it's axis with no wobble, though precessional forces should,at least in my mind) cause the bullet to scribe a spiral path on it's way downrange as well. I think if anything if one is worried about 'overstabilization', a person probably isn't getting the most out of the barrel/bullet combination.

Others may disagree and I'll be happy to learn from my betters. -Rod-

 

[skipper]

There is a lot of good information on Dan Lilja's web site. One of his articles states that you should use the slowest twist that will just stabilize your bullets. There is pleanty more good information for free. Some of his calculations are a little over my head but they make for interesting reading.

http://www.riflebarrels.com/articles/bullets_ballastics/bullet_imbalance_twist.htm

 

[tightneck]

Over-stabilize is not the right term. Bullets can be over-spun and cause erratic accuracy as well as disintegration of the bullet in mid air. I don't think you will have any problems with a 10 twist and 65gr bullets though.

 

[Spotcheck_Billy]

You should be fine with a 1/12 twist and an 85gr. NP bullet. If my memory serves me right Nosler designed ths bullet purposely for hunters using the old 1/12 twist .244 Remingtons.

I'm not so sure about overstabilization but I've got first-hand experience with overspinning a bullet. You should see what happens to at least two out of five 105gr. A-MAX bullets when fired from a 1/7 twist barrel at over 3000 MV

 

[Aussie_bob]

If you want to calculate a twist rate for a particular projectile or minimum velocity etc try useing this.Does it all as long as you put in the correct figures or the particular outcome you want and work backwards.

 

[wildcat284]

Twud,

I've shot the 65gr V-Max in my 1:10 twist Sako .243 varmint and it has been grouping a consistent 1/2 MOA. However, my rifle now shoots the 70gr Nosler BT exclusively as I am getting consistent 1/4 MOA for 5 shots. I have not tried shooting the heavier projectiles, but am looking at working up a 100 grain load for deer at some stage.

Michael.

 

[mikecr]

Aussie
I know you're trying to help but that spredsheet is way out to lunch. Nothing about it is even close to correct.

 

[tdking]

Aussie
I know you're trying to help but that spredsheet is way out to lunch. Nothing about it is even close to correct.

Mike, what about the spreadsheet is "not even close"? I tested it using some simple cases to see if the results made sense and it seems to work okay. I did not take the time to derive the equations that were used, but am curious how you know this spreadsheet is wrong?

Theron

 

[Aussie_bob]

Well Mikecr i hope you have something to back that statement up or are you just blowing wind. Take a trip to this site and explain to them why it don`t work http://www.nfa.ca/ballistics/bullet-stability-calculator-ms-excel.html
Other than that i have put the information from most of my rifles etc through it and stacks up for me especially when you are getting to the edges of stability as in 30BR 18twist and wanting to shoot slightly longer projectiles but the information you get out is only as good as what you put in.
I have just come across some more information and some deals with OVER STABILIZATION and they state that its only noticable in Long Range shooting where the projectile is spun too fast and stays in its nose up attitude and enters the target base first or side on. http://www.fulton-armory.com/fly/stab.htm

 

[mikecr]

I'm not going to try & prove a rule of thumb wrong.
I can see how it would appear to work. Alot like Greenhill,
with similar problems common to 'local' rules:

-It usually calls for too high of a twist than actually needed.
-It relies heavily on velocity to make up for twist rate, which it doesn't.
-It doesn't take air density into account, or a bullet's drag at velocity.

So it doesn't predict bullets like a 115smk stabilizing from an 8tw or even 9tw as shooters are.

Anyone on the edge of stability could define on paper what works and doesn't for those conditions. And this would appear to work in alot of cases elsewhere. Hell, we could have a different rule for every bullet & condition really. And all would appear to work for SOME other cases. But none would work for all.