What determines twist rate?

[WestKentucky]

I understand what twist rate is, and what it does, but why do makers pick certain twist rates for a given caliber? Is there something significant aside from real world testing that actually makes sense? This bore size and this velocity requires X twist rate… Or X RPM to stabilize a bullet Y weight? 1:7 for fast and light bullets in an AR. That’s 333000 rpm. But for a 32 long revolver 1:18 and 1/4 of the linear velocity so we are at 32000 rpm. That’s a factor of 10 in difference, yet both are known to be quite accurate within their normal use parameters. You can easily add in all kinds of other examples that fall in the middle, probably some more extreme on both high and low ends.

Why would a 1:10 .311 7.62 barrel not work with the 32 long? And why would the 1:18 barrel of the 32 not work with the AK?

I’m fully understanding that rifling essentially is an intention impartation of a gyroscope effect on the bullet, but I don’t understand the mechanics of why some bullets need to spin at hundreds of thousands of rpm when others spin significantly slower but are still quite accurate.

Please, if anybody knows the magic behind such twist rate wizardry, could you help a fella understand it?

 

[BreechFace]

It’s all about the length of the projectile and caliber dimension, not the weight.

 

[Mk-211]

Umm what?

Bullet weight not a factor? You do understand that twist rates are made for lighter and heavier bullets, right?

 

[ms6852]

As @BreechFace stated it is about the length of the bullet. The longer the bullet the more it weighs. For example a nato 7.62 round can be 7.62x54 used by the Russians,or 7.62x51 used by Nato countries and the 7.62x39 used in the AK-47's style firearms. All have the same diameter but each have various lengths which have a direct effect on the bullet weight.

The longer the bullet the faster the spin rate has to be to keep it from wobbling, a slower twist rate will make a longer bullet yaw or oscillate giving us what we most term a keyhole on a paper target.

Bullet design also have a great effect where two bullets of same weight and diameter will act different on a certain rifle twist because one of the bullets may have more surface to contact the lands and grooves which is coined "bearing surface".

 

[Tom Myers]

Calculate Twist Rate, Bullet Length and Muzzel Velocity for optimum bullet stability​

This algorithm and the constant 3.5 was suggested by Charlie Dell, in his book, "The Modern Schuetzen Rifle" and brought to my attention by "Tailhook" a member of the MSN BPCR discussion forum.
The constant "3.5" may be re-calculated to conform to known twist-length-velocity combinations that have been proven to produce bullet stability. ​

Define the Variables:
T = Twist rate in 1 turn per inches.
L = Bullet length in inches.
V = Muzzle Velocity
D = Diameter of the bore grooves in inches.
Constant = 3.5
​

Find the optimum twist rate from velocity, bore size and bullet length:
T= ( 3.5 * SqRt( V ) * D² ) ÷ L ​

Find the optimum bullet length from velocity, bore size and twist rate:
L = ( 3.5 * SqRt( V ) * D² ) ÷ T ​

Find the optimum muzzle velocity from bullet length, bore size and twist rate:
V = ( L * T ÷ D² ÷ 3.5 )² ​

Find a different constant to fit known optimim combinations.
Constant = L * T ÷ D² ÷ SqRt( V )​

Hope this helps​

 

[230RN]

As @BreechFace stated it is about the length of the bullet.

That's pretty much it, give or take a couple of things, like the bullet material and the weight distribution, etc.

The best way I can imagine the compromises between bullet weight or length and spin required, is to think about the spin required to stabilize a disc like a frisbee versus stabilizing a long thin object like a broomstick (without fins).

Without going into the angular momentum of gyroscopic stability, the surface speed (rim velocity) of a large object spinning at the same RPM as a smaller diameter object is much greater. In a manner of speaking this makes it harder for a given point on the surface of a larger item to "change direction" in its circular orbit than in a smaller object. In a manner of speaking.

Also the weight distribution of the bullet has to do with stabilization... the more weight in front, the more arrow stability takes effect. Think rifled shotgun slugs. The rifling has almost no effect on spinning.* The point-first stability comes from all the weight in front of the slug. Brenneke, you will note, keeps the wadding attached to the slug to (supposedly) maximize this "arrow" effect. Minie "balls" use both arrow and spin to stabilize the projectile. You can begin to see the variables involved with that simple example.

Calculating the RPM of a bullet is pretty easy if you know the muzzle velocity and the rifling twist, and you don't need to remember any "formula" if you look at the principles involved.

Many bullets exit the barrel with close to half a million RPM. The exact twist for manufacture is actually confirmed experimentally, although approximations can be made with a little arithmetic... as in the "Greenhill Formula."

Terry, 230RN

* You will note that rifled shotgun barrels are/were available to enhance the accuracy of "rifled" slugs. I always wanted to get one for my "500" but put it off until the prices exceeded my need for shotgun slug accuracy.

 

[243winxb]

I dont knows the magic , but ............

*The faster twist will over stabilize a bullet & not cause problems*, unless the bullet blows up on the way to the target.

A to slow twist will no stabilize some long bullet. Accuracy will be poor. Bullet manufactures post required twist rates for some longer then normal bullets.

Mostly a high powder rifle loading issue.

Handguns- Cast or soft lead bullets may skid in the rifling, if twist is fast. Many factors come into play. Soft alloy being number 1.

Note the base of the rifle bullet makes a difference.

https://bergerbullets.com/twist-rate-calculator/

 

[BreechFace]

Umm what?

Bullet weight not a factor? You do understand that twist rates are made for lighter and heavier bullets, right?

People referring to “that twist won’t stabilize that *heavy* of a bullet,” or “that twist rate will spin apart that *light* of a bullet,” is making an ASSUMPTION that heavy bullets are longer and lighter bullets are shorter. Not always the case.

The correct way of discussing twist rates is pertaining to different projectiles given lengths.

 

[12Bravo20]

The more correct way of discussing twist rates is pertaining to different projectiles is length.

M855 and M856 tracer rounds are a good example of this. The M855 bullet is 62 grains while the M856 tracer bullet is 64 grains. But the tracer round is longer and won't stabilize in a 1-9 twist barrel in certain environments so that is why the M16A2 has the 1-7 twist.

 

[jmorris]

Umm what?

Bullet weight not a factor? You do understand that twist rates are made for lighter and heavier bullets, right?

You have to spin the longer bullet faster to stabilize it but yeah, if they are both made from the same materials and alloys, the longer one will weigh more. Really more coincidental than the driving factor.

Take a short bullet made from a heavy material and a long one made from a lighter material though and you still need a faster twist to stabilize the long one despite the weight being the same.

 

[243winxb]

https://jbmballistics.com/ballistics/lengths/lengths.shtml
Here is a list of bullets lengths, by brand. Note the plastic tip is added on some.
Site also has a twist calculator & more.

The target knows best what works.

https://jbmballistics.com/ballistics/bibliography/bibliography.shtml
Or the answer may be here? Winter Reading?

 

[Dave DeLaurant]

My favorite tale of bullet stabilization has to do with the CETME program to design an assault rifle cartridge that would be effective at long range. Ian relates the story in brief, but it's better to read the book if you can find a copy. Oddly, I couldn't find the CETME twist rate in my copy of this book for their extremely long 8mm bullets.

Full Auto at 1000m: The 7.92x41mm CETME Cartridge

http://www.patreon.com/ForgottenWeaponsCool Forgotten Weapons merch! http://shop.bbtv.com/collections/forgotten-weaponsThe US insistence on a full-power rifl...

youtu.be

Twist rate comes under a few related general topics: external ballistics, cartridge intent and design, cartridge-bullet-barrel suitability -- basically a bunch of things involving physics and higher math, which I find interesting but suck at. They're fun to discuss when you have the right audience, but allow me to relate an aside about an experience yesterday with the wrong audience.

I was chatting on the phone with my sister Marie, who said that her husband Rob had read a recent FB posting I made. In that posting I mentioned my new doppler radar chronograph, and the said brother-in-law was perplexed over why anyone would need to know how fast different bullets are traveling.

Understand that this is a guy who holds an MS in physics, was a tenured professor at a major university, and has an international rep for his very advanced lighting simulations. I don't claim to understand how he creates these simulations but he once showed me some formulae; I recall that about 15 years ago one of his simulations was complex enough to bring the Cray supercomputer at University of Indiana to its metaphoric knees. So he's no idiot by any means, just is highly dismissive of my interests vs. his. To be fair, before he married my sister he also once criticized some of my mom's cooking methods in her own kitchen. To this day doesn't think he did anything wrong, so his assuming I'm a knuckle-dragger isn't anything special.

Anyway, I tried to succinctly explain the importance of knowing shot-to-shot velocities to a handloaders. Doing this via an intermediary felt like reasoning with a cat but I did my best. Marie said she would just tell Rob that it made my hobby more enriching, and I suppose that's also true.

 

[HKGuns]

I'm pretty sure the barrel maker determines twist rate.

 

[lightman]

Lots of good info above!
I have known guys that have used formulas to calculate the twist rate when ordering a new barrel. I have had good luck using known info when I've ordered barrels. Its been my experience that faster twist are more forgiving than slower twist.

 

[BreechFace]

Another great example showing the discrepancy of weight of projectile equaling length is copper bullets versus lead core.

There is a common thought/suggestion to go down in grain weight of projectile when shooting copper bullets than one is typically using in a lead core.

One of the reasons (there are others, velocity dependency, greater SD, etc) being is copper projectiles are longer than lead core projectiles when grain weight being equal. So if one is at the max length on a lead core projectile for twist rate of the rifle being used, the copper projectile of the same weight due to its length will likely not stabilize.

 

[230RN]

^"Its been my experience that faster twist are more forgiving than slower twist."

Heh. You're right.

Sometimes I like to take things from minus infinity through zero, and up to plus infinity just to (sometimes) get some insight.

Looking at that, with zero twist there is zero stability, some some twist a better stability, and with even more twist... you guessed it... even more stability.

(Until the projectile flies apart.)

Heh.

Terry, 230RN

 

[illinoisburt]

An interesting side item to the barrel twist affect on flight is the effect it has on terminal ballistics as well. Bullet rotation has much slower degradation in flight than does forward velocity. Therefore we can see measurable differences in bullet upset between a full power round at long distance and a reduced charge at short range. Good example is firing a 180 grain 308 soft point bullet from a 1:10 300 WM at yards versus the same bullet from a 30/40 Krag at 100. Both bullets will be moving around 2200 fps at impact, but the WM is over 212k rpm while the Krag is only 158k rpm at the muzzle. 25% difference in rotational speed means the Krag bullet will experience less upset and is likely to have better weight retention and penetration upon impact. This can be an important component especially for frangible bullets like Bergers which are popular with the long range hunting crowd which may need the additional rpm to upset sufficiently.

 

[BreechFace]

A common thought is faster twist will impart some additional resistance (makes sense) and thus will cause higher or quicker peak chamber pressure. There have been some formulas (I'll try and dig up) where smarter persons than I have attempted to quantify that in terms of projectile weight difference between a hypothetical zero twist rate and how that relates to a given twist rate and projectile weight of a known barrel and projectile. It is something similar to 1:10 twist 30 cal projectile with a weight of 180, if shot through a hypothetical zero twist barrel the projectile that would have similar pressures would be 195gr. Something to that affect. It was determined that there is pressure difference when formulating with physics and mathematics but that it is at a level that is pretty inconsequential to the lay shooter/reloader.

This is where I think a lot of "gain-twist" barrels are thought to gain in a reloaders ability to have resistance build as the projectile travels through the barrel as the twist gains towards the muzzle thus creating a beneficial pressure wave that can be taken advantage of by pushing charge weights higher and thus muzzle velocities.

 

[BreechFace]

I found the post that I was thinking of, I had saved it for further review, also attached is a pdf by the same poster further explaining and providing additional examples.

 

[Mk-211]

Learn something new all the time coming here.

As to Dave's post, I remember about 20 years ago, they had just started using dopler radar to track bullets.

They were tracking bullet flight and velocities, wonder how much that has progressed from then.

 

[jmr40]

All things being equal heavier within the same caliber is longer, but there are plenty of exceptions. These are all 30 caliber bullets, but the bullet on the right only weighs 155 gr, but it is as long or longer than some 175-180 gr bullets. The plastic tips on the bullets on the left make those bullets longer than the other 180 gr bullets.

No pic, but a 180 gr RN bullet will be considerably shorter than any of these.

The large empty space of the Scenar bullet gives you nearly the same BC as most 180 gr bullets, but with the speed possible with a 155 gr bullet. The weight in the rear helps stabilize the bullet and the large open space means you get good expansion on game.

 

[WestKentucky]

A common thought is faster twist will impart some additional resistance (makes sense) and thus will cause higher or quicker peak chamber pressure. There have been some formulas (I'll try and dig up) where smarter persons than I have attempted to quantify that in terms of projectile weight difference between a hypothetical zero twist rate and how that relates to a given twist rate and projectile weight of a known barrel and projectile. It is something similar to 1:10 twist 30 cal projectile with a weight of 180, if shot through a hypothetical zero twist barrel the projectile that would have similar pressures would be 195gr. Something to that affect. It was determined that there is pressure difference when formulating with physics and mathematics but that it is at a level that is pretty inconsequential to the lay shooter/reloader.

This is where I think a lot of "gain-twist" barrels are thought to gain in a reloaders ability to have resistance build as the projectile travels through the barrel as the twist gains towards the muzzle thus creating a beneficial pressure wave that can be taken advantage of by pushing charge weights higher and thus muzzle velocities.

See part in bold. That is consequential in multiple ways, especially to a reloader. When new loaders start asking questions or simply request generic advice from seasoned hands I always give the same reply, which brings about more questions. My reply is that when loading cartridges, you want to load hot enough that the ammo burns cleanly. That is a function of pressure and dwell time as much as anything because under real world scenarios the difference in temperature in various places on earth is somewhat inconsequential to the factors affecting rate of conflagration which are temperature and pressure. -50f at the poles or 130 at the equator is a small change in comparison between pressures inside of a gun barrel during the period of time when a gunshot is actively taking place. Low pressure leads to poor burn which leads to dirty guns, which makes sense, but the flip side of that coin is that a new reloader may try to be extra cheap and load so light that they gets squibs and could potentially blow up a gun due to insufficient pressure causing a barrel blockage as opposed to the more sensible way of blowing up a gun of having an over pressure situation. Additionally clean burn is an indication that the gunpowder is being used within its intended range or “design envelope” as the engineers say, and per my personal experience, produces the best groups on target at the low end of a clean burning load development.

Back to the topic of twist rate, it makes sense that there would be more resistance in a fast twist barrel as opposed to a slow twist barrel. That resistance is very much a necessary evil of the function of a firearm. Resistance causes pressure to spike more quickly. Pressure causes more consistent, more complete, and more clean burn. It makes sense to this fat country boy then that a tighter twist and appropriately high pressure are good things to keep in mind when buying, making, or otherwise using firearms.

 

[OS/N]

Think most were determined by the designers with a particular use in mind and once sammi standardizes it its a done deal.

Some make sense today some don't - newer designs are more about high bcs. Some older chamberings go all the way back to BP days and were never revised. i.e. 38s, 45s, 22s etc... 9mm has a 10 twist and is accuarate with light bullets and the 38s are usually 18 and shoot heavy bullets fine up to 180 grain anyway - they just worked.

Sometimes the ammo and gun makers are not in line either like with the 25-06 - most older guns will not shoot heavier than 100 grain bullets and the ammo makers just want to make heavy rounds for it.

The 223 aint much over 200 yards regardless of the bullet weight and you have all the heavy bullets for it and a fast twist but the 22-250 twist will only shoot light pills - go figure. If the 22-250 twist was fast enough for the 77-80 grains pills you would have a pretty nice longer range shooting rig but it still may not keep up the 26-28 high BC bullets. When the 22-250 was designed there were no 77-80 grain bullets so it didn't make sense to loose a few fps with availble bullets so it would work with something that didn't.

To go outside of sammi specs with the cartridge or chamber or twist is considered wildcating so most producers just stay inside the specs. Its easier to just make a new cartridge that's about the same as an older design and make it better than to go back, change history, and make all existing reference data and gear useless for an established chambering.

All that may just be 1 cent worth.

 

[Walkalong]

It’s all about the length of the projectile and caliber dimension, not the weight.

Yep, and longer ones need to spin faster to stabilize.