Bullet Overstabilization?


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Matt304
August 26, 2008, 11:05 PM
I know that a bullet will not stabilize and shoot accurately from a twist rate which is too slow.

But what about a twist rate which is a little fast for the bullet weights used, will this cause accuracy problems?

I am having a bench pistol barrel made for a .30 caliber cartridge firing light rifle bullets under 140 grains. Recommended twist is 1:12". I was thinking that if someday I decide I want to re-chamber that barrel for .308 with heavier bullets, maybe I should start with a faster twist, like 1:11 or even 1:10. I however don't want to use the faster twist if I will sacrifice accuracy at lighter weights like 135gr.

Thanks

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DoubleTapDrew
August 26, 2008, 11:26 PM
I don't know if it'll cause accuracy problems but the bullet can spin apart en route to the target.

VARifleman
August 26, 2008, 11:27 PM
The bullet can be pointed in a different way than it's going if it's over stabilized (pointed up parallel with the barrel as it heads down) as well as blowing itself apart from spinning too hard.

bogie
August 26, 2008, 11:33 PM
Yeah - overstabilizing a bullet can turn perfectly good groups into "full choke" groups...

Lady with a Gun
August 26, 2008, 11:36 PM
The biggest problem with extra fast rates of twist is the bullet may be prone to self-destruct. In other words, the rate of spin will cause the bullet to fragment.

jonmerritt
August 26, 2008, 11:38 PM
.44 shotgun

Matt304
August 26, 2008, 11:40 PM
I will get the twist for what I am currently planning on using then, only 135gr bullets. So I will go with 1:12.

kragluver
August 26, 2008, 11:43 PM
As alluded to here, there is an upper stability boundary for spin rate as well.

hardhit777
August 26, 2008, 11:52 PM
So what does a bullet that fragments in mid-flight look like?

HH

jnyork
August 27, 2008, 12:18 AM
Kinda like a little puff of smoke, only one I ever saw.

VARifleman
August 27, 2008, 12:51 AM
A blue puff of smoke I've heard.

Loomis
August 27, 2008, 12:55 AM
optimum twist rate also depends on muzzle velocity.

Ridgerunner665
August 27, 2008, 01:03 AM
I'd go with the 1 in 12" twist...the 308 does just fine with that twist up to and including 175 grain bullets...

You are going to want to use 155 to 168 grain bullets out of that 10 - 14 inch barrel on your pistol anyway...the recoil and muzzle blast with the heavier bullets is downright PAINFUL.

Ridgerunner665
August 27, 2008, 01:17 AM
optimum twist rate also depends on muzzle velocity.

True...the 308 does better than most think it will from 14 inch barrels though.

110 grain bullet...2,790 fps.
125 grain bullet...2,715 fps.
150 grain bullet...2,612 fps. (48.8 grains of W748)
168 grain SMK...2,480 fps (46 grains of W748)
175 grain SMK...2,250 fps (42.9 grains of W748)...thats a handful.

MachIVshooter
August 27, 2008, 02:21 AM
So what does a bullet that fragments in mid-flight look like?

Dust.

Try shooting .22 Hornet bullets out of a .22-250 or .220 Swift and see for yourself ;)

As for your .308, 1:10 would be fine. The .308 can't push even the light (110 gr.) bullets fast enough for the centrifugal force to exceed the integrity of jacketed rifle bullets.

Ridgerunner665
August 27, 2008, 02:26 AM
MachIVshooter is right about the 308 not being able to push the light bullets fast enough to fly apart...and 1 in 10" would work...

But 1 in 12" will give you a little more muzzle velocity.

Sylvan-Forge
August 27, 2008, 02:38 AM
Someone noted this site on an older thread ..

http://www.nennstiel-ruprecht.de/bullfly/

I think it's referred to as Tractability ..

.

bogie
August 27, 2008, 03:03 AM
In my experience, when run run 50 grain Speer TNT out of a .22-250 at a max load, even with a 12" twist, you get a contrail of bluish-grey smoke starting past 50 yards... It was making holes in a hundred yard target tho...

Jim Watson
August 27, 2008, 10:35 AM
12" twist is already rather fast if all you are going to shoot is 135 gr benchrest .30s.
It will be fine for anything up to 175 gr.

big_bang
August 27, 2008, 11:20 AM
For best benchrest match accuracy, you want to spin the bullet as slow as possible to stabilize it. Some benchresters will drop one twist rate (1:14 to 1:13 for example) in winter versus summer to stay right on that edge of stabilization.

For most other purposes, as long as spin rate is somewhere between min-max you're fine. Minimum is what required to stabilize, max is based on bullet construction.

brickeyee
August 27, 2008, 11:33 AM
There is no upper limit other than bullet constriction.
Faster spin than needed does NOT introduce instability by itself.

The problem is that if the bullet is not perfect the excess spin will cause problems.

The bullet's longitudinal center of gravity and center of form do not match perfectly.

The bullet travels in the barrel around center of form.
Once free of the barrel it rotates around the longitudinal center of gravity.

Any more spin than needed for stability increases the forces acting while rotating around the longitudinal center of gravity degrading accuracy.

Unless you have a BR grade rifle you are unlikely to see any effect from excess spin, besides bullets simply coming apart.

spwenger
August 27, 2008, 01:13 PM
Faster spin than needed does NOT introduce instability by itself.
This statement would be true if bullets traveled in straight lines. However, the trajectory of a bullet is shaped by gravity. Rifling conveys gyroscopic trajectory to bullets; if the gyroscopic stability is "perfect," it will lead the nose of the bullet to remain on its initial path as the bullet enters the descending phase of its trajectory. The increasing difference in angle between the direction of the nose of the bullet and its actual path will produce "yaw," where the tail of the bullet does not follow the nose of the bullet. This is the source of the inaccuracy induced by "overstabilizing" bullets with too high a twist rate. The effect may not be seen at close range but will magnify as the range increases.

LB7_Driver
August 27, 2008, 01:51 PM
Spwenger has a few points, especially at very long ranges.
Brickeyee's observations tend to hold true under most shooting conditions.

In my experience, it is very hard to over-stabilize a bullet (barring structural limits).
A well-made bullet will not be measurably less accurate when fired from a fast-twist barrel.
Fast-twist barrels will bring out the poor accuracy characteristics of poorly-made bullets more than a slower-twist barrel.

All of the above depends upon rifles of average accuracy... if we are talking about rifles that are capable of consistently printing sub-1/2 MOA groups, then you can start to measure the effects of stabilizing a bullet more than absolutely necessary.

Good bullets, shot from a 1-MOA rifle that has a faster-than-necessary twist, will not be measurably less accurate.

Case in point -
A 68-grain, 6mm HP match bullet is a common bullet style for BR shooters. Most BR rifles have a 13 or a 14 twist barrel for this bullet weight/style.
This same bullet will still print ~1/4 MOA groups from a 1-8 twist barrel.

brickeyee
August 27, 2008, 02:05 PM
This statement would be true if bullets traveled in straight lines. However, the trajectory of a bullet is shaped by gravity. Rifling conveys gyroscopic trajectory to bullets; if the gyroscopic stability is "perfect," it will lead the nose of the bullet to remain on its initial path as the bullet enters the descending phase of its trajectory.

This only occurs at very extended range, not something about 99.9% of shooters are ever going to see.

Yaw does not in itself represent an accuracy loss, but can represent a precision loss.

The group can remain tight, it just will not be in the expected location.

Cross winds are likely to spread the group significantly more.

thrifty7
August 27, 2008, 02:22 PM
I beleive that the original question was,to paraphrase, " would a 10" twist be too fast for a .308?". Although many .308s have a 12" twist, many others have 10". The standard twist for most 30 caliber cartridges over the last 100 years has been 10"----no way is it too fast. All of the "puff of smoke" comments would only make sense if he was talking about something really radical, down in the 6" region---or so it would seem to me.

Vern Humphrey
August 27, 2008, 03:18 PM
"Over stabilization" can result in trail -- the bullet continuing to point in the original direction of launch, instead of keeping it's axis parallel to the direction of motion. That's rare, and usually occurs at very steep angles, so the falling bullet hits the ground base-first (common in anti-aircraft useage.) Beyond that, it has no real effect on accuracy.

High velocity and a high rate of spin can initially over-stress the bullet as it takes the rifling, leading it to come apart in air from centrifugal force -- this used to be fairly common in very high velocity .22 centerfires

lookn4varmints
August 27, 2008, 03:36 PM
I think a very important piece of data is missing, yes? What will the barrel length be on your PISTOL? Are many of us jumping right to rifle twist rates here or I am just totally off? I'm not at all familiar with formal benchrest pistol shooting so maybe I'm missing something in this discussion.

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