Barrel twist vs. bullet weight

[Chas.]

I see a lot of discussion about what weight bullet a certain twist will stabilize.  I have see a few references to the length of bullet that a certain twist will stabilize.  Is it safe to assume that the bullet length is the determinator of what a certain twist will stabilize and bullet weight is just a convenient way to discuss it?  Am I over-analyzing and just confusing myself?

[PowPow]

There is something to it.

Google "greenhill formula" for more than you may want to know about it.

I have seen long bullets do stupid stuff out of long twist barrels.

[LaOtto222]

Bullet length has more to do with it than weight. The newer non-lead bullets are longer than their lead core counter parts. Even though they may weigh the same, it takes a faster twist to stabilize them. To make them able to stabilize in a standard twist barrel, they are generally made in a weight that is lighter than a lead core bullet. Another example: a heavier lead core bullet may stabilize better than lighter one depending on bullet shape. A flat based round nose bullet will stabilize easier than a VLD type bullet. A .308 200 grain round nose will stabilize in a slower twist barrel than a 180 grain boat tailed spitzer, because the bullet is shorter. Why does a .430 bullet that weights 240 grains stabilize in a 1 - 38" twist while a .224" bullet that weighs 80 grains will not stabilize in a 1 - 12" twist?  Sure bullet diameter has a lot to do with needed twist rate, but that is a dramatic difference. Centrifugal forces on the outside diameter increases as the diameter increases and helps stabilize the bullet so less twist is needed, like a top spinning. If the .430" bullet gets too long, even it's diameter will not be enough. Length and diameter makes the difference, not weight.

[quickdtoo]

Here's Lilja's general recommendations on twist rate and bullet weight. There are some bullet styles that are short for their weight and work fine in slower twists, a good example in .223 Rem are the 63gr Sierra Varminter and 64gr Win PP which are short for their weight and work fine in 1:12" twist barrel which other bullets of the same weight may not. At the other end of the scale would be VLD bullets that are very long for their weight, Berger shows the twist required for their bullets.

Tim

http://www.riflebarrels.com/products/caliber_twist_rates.htm

http://www.bergerbullets.com/Products/All%20Bullets.html

[Dand]

Doesn't velocity work into it too?  I think a long bullet won't stablize with a marginal twist and low velocity.

Crank the speed way up and it might stabilize.

Didn't some older 300 weatherby's have a 1:12 inch pitch while most 30 cal barrels are 1:10 inch?

[41 mag]

Doesn't velocity work into it too?  I think a long bullet won't stablize with a marginal twist and low velocity.

Crank the speed way up and it might stabilize.

This also goes back to the lenght and twist required. IF you do have a marginal bullet for twist and you CAN speed it up, your actually getting more RPM's which does more for the stabilizing than the velocity.

However, you can have a fast enough twist and low enough velocity that the bullet will still do weird stuff. My daughters 6.5x55 shoots the 140gr A-Max into nice little clover leaf groups at 100yds. At 200yds the groups open up to around 1.5" on average but they hit low and right. Move out to 300yds, and they are still within around 2.5" but they are now grouping around 9 o'clock. They are on the ragged edge of the speed and as such as they go down range they start to spiral in an outward circle.

I ran this by Charlie Sisk of Sisk Custom rifles on day while out shooting some loads through it. He said he had seen it in several rifles using the heavier or longer bullets for caliber while working up from start loads.

I figured either way, it is still withing what we needed for the hunting we do with it which is generally under 100yds so we kept the load and have been happy with the results.

[helotaxi]

The difference in velocity required to make up for a marginal twist rate usually isn't achievable. 

The length of the bullet isn't the determinant factor and neither is the diameter.  The ratio of the length to diameter is what determines the RPM that the bullet must spin at to be stable.

Long bullets do funny things, period.  If they are just barely spinning fast enough to be stable, they will tumble end over end once they strike something.  This is the wounding mechanism that the killing power of the Whisper series of subsonic rounds is based on.

[torpedoman]

you can't change the dia of the bullet so as weight goes up it has to get longer (assuming copper/lead bullet) so weight and length are related  fps figures in and an increase in velocity will help to stabilize the bullet.hornaday 204 40 gr would not stabilize in my handi after polishing the bore and molyfusion treating it the fps increased  and it now stabilizes the 40 grain bulllet.

[Dezynco]

OK, I'm following along this thread.  I've always understood that longer, heavier bullets require a bit faster spin to stabilize.  I had two Sharps replicas in 45-70.  One had a 1-20" twist, the other had a 1-18" twist.  Both rifles shot fine - until I loaded with anything heavier than about 500 grains.  525 grain bullets would shoot great from the 1-18" twist, but would not hit the side of a barn in the other rifle.  Generally speaking, that follows the "rule".

One thing that I've always wondered, but not seen mentioned on this thread....

A 300 Whisper (for example) has a 1-8" twist (I think this is what the 300 Whisper is supposed to have to stabilize the heavy 200+ grain bullets).

What happens if you fire 90, 100, 110's that are on the very light end of the .308 caliber?  Do they "over-rev" and become unstable?

[PowPow]

I have read that shooting a short bullet in a short twist "over-stabilizes" it.
Its possible to spin it above its "come apart" speed.
Or the imperfect concentricity makes it wobble worse at high speed, like a car tire out of balance.
I have slowed them down with powder selection or charge to avoid that.

 

[AtlLaw]

I have read that shooting a short bullet in a short twist "over-stabilizes" it.

Can you "over stabilize" a bullet?   
Or is it the bullet is either stabilized or not?   
If not stabilized it wobbles/tumbles/does something to show it is not stable.
what does an "over stabilized" bullet do?   
Not including coming apart because of extreme centrifugal force.   
I wonder about this every time I hear "over stabilized."   

[Scibaer]

can you over stabilize a bullet ? yup..

 "An over-stabilized bullet rotates too fast and its axis tends to keep its orientation in space" ...

meaning  "The bulletīs longitudional axis becomes uncapable of following the bending path of the trajectory"

"An over-stabilized bullet tends to land base first,  experiencing yaw on the trajectory"

but also, "when firing bullets from handguns, over-stabilization is of minor importance in normal shooting situations"

quotes taken from a article on ballistics, of which i've found more and more interesting all the time, if not useful

[PowPow]

AtlLaw,  I will rely on Scibaer's explanation. All I had to offer for "over-stabilized" was "living in a hay loft for a long time".

[AtlLaw]

"over-stabilized" was "living in a hay loft for a long time".

 

[Dand]

Seems like in the last couple  years Rifle or Handloader had some articles on the (?? 223) and twist rates from 8 to 12. I think it concluded that todays bullets are much higher quality, concentricity than 25-30 yrs ago and that "over stablizing" may not be as much of an issue these days. I think for the "normal" range of bullet wts in .223/.224, say 45 or 50 gr to 60 /62 gr it was more desirable to be slightly over stablized than to risk under stablizing. I understood that meant you should be fine with say 9 or 10 in twists than 12 or 14 unless you were looking for the very last nit of accuracy out of a particular bullet wt. in a particular cartridge.

[helotaxi]

Spinning a bullet faster than it needs to be spun will not make it unstable.  It will be quite stable but it might wobble because of imperfections in the bullet related to CG and axis of spin.  If the CG is not somewhere exactly on the spin axis, the bullet will begin to wobble as soon as it leaves the barrel.  The faster you spin anything, the more exaggerated any imperfections are.  As was hinted at in the previous post, current quality bullets are better than in years past in regard to concentricity and uniformity making the likelihood of spinning one into a serious wobble less likely.  Cheap bullets are still quite susceptible.  Benchrest shooters typically go with the slowest rate of twist that will stabilize the bullet they plan to shoot to minimize any chance of a wobble.  Many times their bullets are at the ragged edge of being stable and a slight decrease in muzzle velocity will keep them from stabilizing.

Spinning a modern bullet so fast that it comes apart in flight is difficult but not impossible.  It requires a very fast twist barrel or a very high muzzle velocity and a very lightly constructed bullet.  The bullets designed for the .22 Hornet can be grenaded in flight fairly easily with a .22-250 or a 1:7 twist .223.

The .300 Whisper will shoot any bullet that you can load in the case quite accurately.  Even light bullets loaded to supersonic velocities are not going to be spun as fast by that 1:8 barrel as they would by a .308 or .30-06 in a 1:10 barrel because there is a very large difference in velocity.

[PawPaw]

When you get into the cast bullet game, things get even more interesting.  One of the finest cast bullets for .30 cal rifles is the Lyman 311041, which is a 170 grain flat nosed bullet, used for such things as the .30-30.  Shooting it through a standard 1:10 twist barrel (which is the basic standard for .30 cal barrels today), it does well up to about 1800 fps or so.  However, many of us, for pure accuracy, shoot it at about 1500 fps.  If you jack the powder up and travel it at 2200 fps, accuracy goes out the window.  Many of us theorize that the bullet is being pushed too fast and is starting to strip in the rifling .  Others of us theorize that the bullet is never truly concentric and the additional rpms are making it unstable.

However, if you can find an old 1:12 or 1:14 barrel, the accuracy gets back to the better level. 

at 2000 fps, a bullet going through a 1:10 barrel is spinning 14,400 rpm.
at 2000 fps, a bullet going through a 1:12 barrel is spinning 12,000 rpm.

If my math is correct.  When you're playing with cast bullets, barrel twist becomes very important.

Nope, that's wrong.  I forgot to carry a zero.  The formula for calculating bullet rpm is
RPM = MV * 720/twist rate, where MV is the muzzle velocity.


at 2000 fps, a bullet going through a 1:10 barrel is spinning 144,000 rpm.
at 2000 fps, a bullet going through a 1:12 barrel is spinning 120,000 rpm.