Bullet stabilization

[Doug Lee]

Hello All,
I have long been puzzled by the external ballistics concept of bullet stabilization and rate of twist of the barrel. An example would be a .25-06 with a 1 in 10 inches rate of twist firing a 115 grain bullet verses a 75 grain. Can someone explain the bullet stabilization concept?

 

[PawPaw]

Not without a huge explanation and lots of caveats. The answer tends to run to the optimal twist for a particular bullet, based on that specific bullet's center of gravity, length, ballistic coefficient, and internal construction. There are entire chapters in books devoted to the little mysteries of bullet stabilization.

However, the short answer is that yes, you can fire a 115 grain, .25 caliber bullet in a 10 twist barrel, and you can fire a 75 grain bullet in that same 10 twist barrel.

I personally don't believe that you can "overstabilize" a bullet, but I do believe that you can "over-rotate" a bullet. I've seen .22 caliber bullets come apart on the way to the target. A lightweight, poorly constructed bullet, pushed too fast and spun too fast can disintegrate as it goes downrange. I've seen it happen.

 

[jwrowland77]

I can't explain it, but know it has a great deal to do with the length of bullet, muzzle velocity of said bullet along with the twist creating them rpms needed.

Here's a couple of links that are very useful:

Stability calculator.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Bullet lengths.
http://jbmballistics.com/ballistics/lengths/lengths.shtml#

I use a 175gr SMK in a 1:12 twist which charts say is no good but due to length of bullet, it works great.

 

[PawPaw]

I use a 175gr SMK in a 1:12 twist which charts say is no good but due to length of bullet, it works great.

There's that too. Sometimes the charts tell us that a particular bullet/twist combination just won't work, but we go ahead and try it anyway. And it works for us against all the advise of the charts. The one thing I've learned in my experience (and my decidedly un-scientific approach to handloading) is that every barrel is different.

 

[Doug Lee]

rule of thumb

Thanks for the feedback, I guess i was after a rule of thumb, not to be confusing, like heavier bullets do better in a slow twist, or lighter bullets do better in faster twist rates.

 

[jwrowland77]

Thanks for the feedback, I guess I was after a rule of thumb, not to be confusing, like heavier bullets do better in a slow twist, or lighter bullets do better in faster twist rates.

It would be, heavier bullets in faster twist, lighter bullets in slower twist.

 

[PawPaw]

But, Doug, if it helps, I routinely shoot 115-117 grain bullets in my 10 twist .25-06 rifles. The family has four of them and they handle the heavy bullets very well. Because I load for four of those rifles, and I'm never sure what rifle the particular cartridge might be fired through, I've come up with what I call my "standard load" and it's in the middle of every chart I've ever seen. I use 50.0 grains of Reloder 22 with a WLR primer, under a 115 Gameking or a 117 grain SST. That load shoots under MOA in each of the four rifles and is very satisfying both on the range and in the field.

 

[Jimro]

Well, to through a monkey wrench in to the mix.

The actual RPM of the bullet is determined by (muzzle velocity in inches/second) divided by (twist in inches) then multiplied by 60 to get rotations per minute.

That means if you aren't stabilizing a bullet because your twist is too slow, you can change to a faster twist barrel, or increase muzzle velocity.

You see this in the AR world with heavy match bullets and a 1:9 twist. Once you get past 20" barrels for space guns, the bullets are launched fast enough that they can achieve stability. The difference between what a 16" 1:9 twist barrel and a 20" 1:9 twist barrel will handle is pretty interesting too.

Anyways, the 25-06 with a 1:10 twist should be fine with the 115 bullets, unless for some reason the barrel is insanely short.

Jimro

 

[Doug Lee]

Rule of Thumb

Thanks for the feedback . . . jwroland77 said simply , "heavier bullets in faster twist, lighter bullets in slower twist".
Yesterday, I shot a friends .25-06 Mossberg model 800 rifle fitted with a Douglas (XX) barrel. I shot 3 rounds slow fire with 115 grain bullets with about a 3 inch spread to the group at 125 yards, then 4 rounds repeat fire with 75 grain VMax with about 1 - 1.25 inch spread to the group, it seems the lighter bullets are more accurate when fired from that rifle. I was excited on the second group, that's why the repeat fire, normally I use more control.
Another reason for the questions is, I have a Ruger Model 77 VT that I can't get to group very well, usually 2.5 - 3.5 inch groups, but this Mossberg 800 with the Douglas Barrel does really well, maybe I should try to work up a trade with the gal that owns the Mossberg 800.
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[JD0x0]

Anyways, the 25-06 with a 1:10 twist should be fine with the 115 bullets, unless for some reason the barrel is insanely short.

...And/or the bullet is insanely long. (Think all copper VLD bullets)
The berger 115 VLD looks like it'll just barely stabilize in a 1:10 25-06 @ 3000fps, depending on temperature. Colder temps will make bullets less likely to stabilize. Berger's twist calculator gives it a stability factor of 1.14 at 0 degrees Fahrenheit(1.31 @ 65 degrees). 1.0 being unstable, 1.4+ being 'comfortable stability'
The Barnes TTSX in the same weight are even longer at 1.224'' (The berger being 1.2'') and are the longest .257 cal bullets I've found in that weight.

 

[USMC 77-81]

Just something for you to think about, the thing that stabilizes the bullet which is rotational spinning eventually destroys accuracy, I first realized that watching night fire with tracers in the Marine Corps. Keep in mind that bullets look concentric but if miked down far enough you will find they are eccentric, when spun the eccentricity creates 'Throw' which is why they then go off into never never land, just saying for your general knowledge.

 

[Jimro]

USMC77-81,

Tracers are an interesting ballistic problem, they lose mass as they go along, but they also reduce drag because that mass is filling the main drag cavity with hot gasses. If the tracer compound isn't burning perfectly concentric, you get a serious out of balance situation where they go flying off in odd directions as you saw.

Normal bullets don't lose mass, so the only thing that a tighter twist will do is slightly open up groups by a very minor amount. Not something that comes into play with anything other than benchresters though. It is true that spinning a bullet as slowly as you can and still achieve stability is the combination that will produce best accuracy, but even overspinning a bullet won't open groups up more than a very small fraction of a minute of angle.

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

Jimro

 

[tangolima]

Gyroscopic stabilization can be rather quirky if you dig deep into it. But for hand loaders, like myself, a simplified version mostly suffices.

1. Long, pointed bullets need to spin faster. This is because the point of pressure is farther ahead of center of gravity, i.e. they tend to stumble more than their stocky friends.

2. Heavy bullets tend to be long, so they usually need faster twist rate. Bullets made of less dense material, lead-free, will be even worse.

3. When in doubt, go for a faster twist.

4. You can't over "stabilize" a bullet, but when rpm goes over certain value, there is no further benefit as other factors become dominant. The "side-effect" of high rpm will certainly start causing problems.

5. Sir Greenhill came up with an empirical formula, the Greenhill's equation, to determine needed twist rate, more than 100 years ago. It still works today, at least for a ball-park figure. Try it on your favorite bullet to see whether it makes sense http://en.wikipedia.org/wiki/Alfred_George_Greenhill.

-TL

 

[Doug Lee]

Rule of Thumb

Hello All, Thanks for all the interesting feed back . . . I can clearly "get" the info tangolima is responding with . . .Thanks . . .
Doug Lee
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