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Bullet stability factor: big deal or not

 
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Old 08-05-2010, 12:38 PM
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Bullet stability factor: big deal or not

GS Custom makes a big deal out of bullet stability factor. And I believe they are a leader in bullet design for long range and other applications. For instance they are building some of the longest bullets made, 5-7 caliber, like the 414 grain 375 caliber.

GS recommends: SF > 1.6 for dangerous game bullets at close range, SF > 1.4 for normal hunting up to 500 yds, and SF between 1.1 and 1.5 for long range. The later is so that the bullet will nose over at long range. So if we spin our bullets as needed for long range accuracy, they may not be stable enough if they hit a heavy animal.

My thinking is that this probably isn't as important for a bullet that expands fairly easily like the 300 SMK but could come into play on a bullet like the 265 TTSX, both 338 caliber.

What do you think? Why don't the other bullet makers give this info? Do I need to be concerned if I load a 150 TTSX in my 7mm 10 twist? I would just be guessing on the SF for the later.
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Old 08-05-2010, 10:55 PM
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Re: Bullet stability factor: big deal or not

Hey guys I'll take another shot at stating what I'm thinking.

I'd say we're likely giving up terminal performance of our bullets in some cases by giving them minimal spin to gain accuracy at long range.

The way I see it the guys that shoot cape buffalo and the like are the best experts on terminal bullet performance. They are going to 10 twists for bullets that are flight stable in 16-18 twist in say 458 and 510 caliber. This is for better straight line penetration on bullet impact.

This has to have some implications for some of our bullets that we give minimal twist. Just because our game isn't dangerous or thick skin, I would still think hitting heavy bone or whatever might be a problem.

In addition even for close range hunting with the long monometal bullets like Barnes I'm concerned that they might be flight stable but not impact stable. If I take a 150 TTSX against elk that's as much bullet penetration stress as a 300 TSX against a hippo.
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Old 08-05-2010, 11:48 PM
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Re: Bullet stability factor: big deal or not

Stability does not decrease over time. It actually gets better until it hits the transonic zone, then destabilizes (much of the time anyway).

Also, other bullet makers dont offer that info because they make traditional bullets that will stabilize in traditional twists. Although, Sierra makes note that certain bullets will need a specified twist to stabilize.
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Old 08-06-2010, 06:13 AM
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Re: Bullet stability factor: big deal or not

Bullets are designed differently to do different jobs.

Bullet stability affects what a bullet does after impact and you should decide up front how you want the bullet to behave and get a barrel twist that provides the spins you wish at the ranges you wish.

When considering spin and impact behavior you can loosely segregate the bullets into two designs, nearly symmetrical and asymmetrical. You then must subdivide those into expanding and non-expanding bullets. Stability is achieved by different means for the different bullets and spin may be of little importance in some of the categories. Even in the same category more spin and less spin will give you different results upon impact.

At extreme ranges less spin may well be a better choice however even then the cat may be skinned by excess spin depending upon other factors of bullet design.
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Old 08-06-2010, 07:16 AM
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Re: Bullet stability factor: big deal or not

so now just so i have this straight. so that next time im standing there with a cape buffalo charging, which gun with which bullet should i ask the gun bearer for?
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Old 08-06-2010, 08:01 AM
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Re: Bullet stability factor: big deal or not

Short answer----300 yds and less use a FB. This question has been covered many times do a search.
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Old 08-06-2010, 08:49 AM
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Re: Bullet stability factor: big deal or not

yobuck:

I think you're pulling my leg . But just in case you're not I'd ask for the 505 Gibbs shooting 500-600 grain Barnes or Northfork flat nose solids in a 10 twist barrel. The flat part of the nose would be 65% of the diameter of the bullet. Those bullets or TSX types would fly straight thru the air in a 20 twist. But the guys shooting dangerous game want the bullet spinning much faster so it penetrates in a straight line.

I know this is a lot different than long range shooting. But I do think it has implications for long range especially when shooting something like the 265 TTSX in 338. I'd like taking on a buffalo with that bullet. But I'd have it spinning much faster than we would for 1000 yd shooting.

Last edited by Varminator 911; 08-06-2010 at 09:53 AM.
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