BARREL LENGTH AND TWIST RATES DISCUSSION

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BARREL LENGTH AND TWIST RATES DISCUSSION

Postby 3rdgeargrndrr » Sat Nov 24, 2012 10:01 am

Hey guys, I did see this briefly discussed in another thread, but wanted to have a longer discussion about it. I know this might be somewhat of a black art, but I wanted to know your thoughts on barrel length and twist rates.
I'm leaning toward Mcgowen to build this barrel.
Bushmasters twist rate on the AR barrel is 1:24. This is a very accurate barrel from the get go, however out of a bolt gun, you have some more flexibility in developing loads. and to tailor to suit specific needs.
Twist rates for the 45acp, 45LC, 460 SW, 450 marlin, 458WM range from 1:14; 1:16, 1:18.75, 1:20, 1:22, 1:24.

the Greenhill equation (twist calculator) gives me an impossibly long 1:40 to 1:64 twist rate depending on load. (smoothbore!)

With these twist rates, now barrel length comes into play. What are your guys thoughts on the correct length to fully stabilize a bullet?
Powders used on the 458WM look like they go as slow as Benchmark for 300gr bullets, and Varget in the 350gr arena. Which should be fully burned by probably an 18" barrel with that heavy of a projectile.

I would be under the impression that a for me primarily shooting 300grainers @ max of 2400fps- 1:20 or 24 twist with a 24" bull barrel might be the best choice.

And even before I get started with this build exercise, Tim, any idea on where to find a Legendre Magnum reamer? :lol:
Last edited by 3rdgeargrndrr on Sat Nov 24, 2012 10:16 pm, edited 1 time in total.
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby pitted bore » Sat Nov 24, 2012 4:25 pm

3rdgeargrndrr wrote: ... I am in the process of rebarreling my 30-06 to 308 now, but might change my mind or start a new project. ...

3rdgeargrndrr-
Is this a discussion of 308 caliber? Or 452? I'll admit readily to being an individual who is easily confused.

3rdgeargrndrr wrote: ... I'm leaning toward Mcgowen to build this barrel. ...

Is McGowen going to be building a 308 barrel or a 450B barrel or 450LM barrel for you ? ? ?

Let's assume you want a discussion of twist rate for a 450B or 450LM, with a caliber of 0.452. My understanding of the twist problem is limited, but I think you should be able to find an appropriate twist by knowing answers to these questions:
  • What is the minimum temperature at which you plan to shoot?
  • Do you plan to shoot where air pressures are higher than sea level, like on the shore of the Dead Sea? If so, what's the barometric pressure?
  • What are the length and weight of the longest bullet you ever intend to shoot?
  • What is the least muzzle velocity (fps) at which you ever plan to shoot this bullet?
With answers to these questions, it's possible to get a pretty good idea of the twist needed for adequate stability to enable accurate shooting.

One of the better on-line twist rate calculators is that at jbm: jbm ballistics The articles from Precision Shooting that are given in the bibliography for the Stability calculations are pretty informative on the subject of twist rate.

Using the jbm calculator, let's plug in some values for the shortest 450B bullet I've got on hand: a Prvi Partizan 185-gr jacketed hollowpoint that is 0.525 inches long.

Let's use the default values for temperature (59F) and pressure (29.92mmHg). If we plan on a pretty slow muzzle velocity of 2000 fps, the calculator says we need a twist rate of 1 turn in 55.6 inches to obtain a spin needed for stability. More velocity well result in requiring less twist for stability.

For a worst-case trial, we can insert the values for the longest 450B bullet I've got on hand, a 300-grain Hornady flex-tip at 1.075 inches length. Using the same 2000 fps, the calculator says we've got to have a twist rate of at least 1-in-29.4 inches for stability. A lesser rate (for example, 1-in-30) provides marginal stability. A rate of 1-in-24" provides plenty of stability.

The subject of defining "adequate", "marginal", etc., is complex, and will depend on multiple factors. See the articles referred to above on the jbm site.

3rdgeargrndrr wrote: ... With these twist rates, now barrel length comes into play. What are your guys thoughts on the correct length to fully stabilize a bullet? ...

Perhaps I'm not understanding your statement. Barrel length does not influence bullet spin and stability, except as more length may result in higher velocity. If muzzle velocities are equal, stability and rate of spin will be the same, regardless of barrel length.

3rdgeargrndrr wrote: ... I would be under the impression that for me primarily shooting 300grainers @ max of 2400fps- 1:20 or 24 twist with a 24" bull barrel might be the best choice. ...

Let's plug your values into the jbm calculator: Assuming temperature and pressure defaults, and if we use the 300-gr Hornady FTX bullet, a 1:24 twist will certainly stabilize the bullet at 2400 fps. In fact, if the calculator is correct, that bullet will be stable at a subsonic 1000 fps.

If I've used the calculator correctly, you could swage a 500-grain jacketed SP .458 bullet to .452 at 1.35" length, and at 1400 fps MV still have stability with the 1-in-24" barrel.

So, unless I've messed up above (Quickload sharks should feel free to chew on me), select the McGowen 1-in-24" barrel with no qualms about bullet stability. If I've not addressed your question adequately, please try again.
--Bob

Footnote question for you: Do you have, or know of, any published and reliable 450B loading data what will produce 2400 fps with a 300-grain bullet in a 24" barrel? What are the pressures involved in producing this? Thanks.
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby 3rdgeargrndrr » Sat Nov 24, 2012 10:43 pm

My apologies Bob for the confusing statement. I edited it and appreciate your input and expertise on the subject. I deleted a statement that I originally wrote stating that I was reconsidering the 308 and moving up to 450 BM instead to clear that up.
Length: In regards to "fully stabilizing a bullet" I was told on another forum (sorry you guys can burn me now) that there is a "proper" barrel length to achieve bullet stabilization. Ok it really made sense at the time. But I was also hoping that a couple of inches would also increase the range of the projectile but we will have to wait and see what final velocities are attainable first of course.
Twist: I understand that twist rate has little to do with weight but is affected by length. Using the calculator, very long twist rates would be ideal however Mcgowen blanks come with longest twist rate of 1:24. I can probably ask for longer but I feel that I would be moving away from standardized loads with known twist rates.
I probably know just enough to be dangerous at this point.

Footnote answer. ~74,000 cup. 2400 fps is not obtainable but ~2200 is at just over the limit of sane/insane. Thanks for the reality check :oops: All these guns start with a dream right?
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby pitted bore » Sun Nov 25, 2012 1:16 pm

3rdgeargrndrr wrote: ... Length: In regards to "fully stabilizing a bullet" I was told on another forum (sorry you guys can burn me now) that there is a "proper" barrel length to achieve bullet stabilization. Ok it really made sense at the time. But I was also hoping that a couple of inches would also increase the range of the projectile but we will have to wait and see what final velocities are attainable first of course. ...

3rdgeargrndrr-
If the discussion in the other forum involved stability relative to bullet spin, then I'm skeptical of "proper length". There are lots of ideas about barrel length required to obtain best accuracy, but those ideas are generally unrelated to rate of twist. Benchrest competitors like stubby barrels for various reasons. They also usually shoot with barrels having relatively slow twist rates, but as far as I'm aware the two preferences are not directly related.

Wildcatter/Tim will have experimental information on the velocity gains with increasing barrel length. The couple of inches between 22 and 24 probably won't make a lot of velocity difference with this cartridge.

3rdgeargrndrr wrote: ... Twist: I understand that twist rate has little to do with weight but is affected by length. Using the calculator, very long twist rates would be ideal however Mcgowen blanks come with longest twist rate of 1:24. I can probably ask for longer but I feel that I would be moving away from standardized loads with known twist rates.
I probably know just enough to be dangerous at this point. ...

Bullet length is the dominant factor in calculating stability, but weight/density also plays a part. If you play with the jbm calculator, and hold everything constant but the bullet weight, you will find that stability decreases as weight decreases. (The gyroscopes that kids (used to?) play with have a pretty heavy wheel. and the wheel on gyro compasses is heavy.)

While stability obviously can be obtained with slower twists than 1:24 with this caliber (Wildcatter has used 1:45 for some experiments, I think), I'm not sure those twist rates are "ideal". The 1:24 is a pretty good compromise.

3rdgeargrndrr wrote: All these guns start with a dream right?

Oh yes!
--Bob
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby Gruger » Mon Nov 26, 2012 3:28 am

Some time ago i was playing around with twist rates and resulting spins.
My idea was: We know, that 1:24 and Hornady FTX (250grain) at 2200fps is accurate.
If spin / stability is that important, how do i duplicate the accuracy of the original setup with other twists? What spin rates are standard in other calibers? and so on..

spin.png
spin.png (35.52 KiB) Viewed 30982 times


As you can see the conclusion is: If you change twist rates to pistol caliber rates (1:16 or 1:18) you have to decrease velocity to duplicate the spin rate of an original BM barrel. With 1:18 or 1:16 the decrease is way to high imho.
Spin rate is a function of velocity and twist. The bullet weights are only informative, i took the data from the hornady reloading handbook and other ressorces and wanted to be clear which data i was using.

With 1:20 and 1925 fps you are at 69300 spin and thats about the spin of a 240gr XTP Mag at 2300fps out of a Bushmaster 1:24 barrel.


Still, i'm absolutely not sure if "over stabilizing" a bullet will have that much effect on accuracy.
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby pitted bore » Mon Nov 26, 2012 9:25 am

Gruger-
Thank you for posting an informative and thought-provoking table. I hope you will be able to answer a couple of questions about the information in the box in the upper right-hand part of the table?

I think that your first formula, (150)*(dia^2)/(Bullet length), calculates "required spin". (Is "spin" the rate of twist?) The second formula, (150)*(dia^2)/(twist rate), calculates a bullet length.

What I do not understand is the value of 150. Could you describe its derivation, or post a link so that I can read about it?

I'm also curious which bullet you used for the length of 0.922 inches. The standard Hornady 250-grain bullet used in their factory ammunition is about 0.955 inches in length.

The information in this article has been helpful to me: Calculating Bullet Spin Rates and Stability. The follow-up comment by Bryan Litz is particularly informative. (Some of the other comments are not helpful.)

Thank you again.
--Bob
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby Gruger » Mon Nov 26, 2012 12:18 pm

Hello Bob,

the 150 is part of the classic greenhill formula to get a rough idea of what twist is required by a given bullet (and caliber) to stabilize it.

Twist = 150 * diameter^2 / (Length of bullet)

So you can buy the right barrel for your chosen bullet(s).

The "max. bullet length" is derived from that formula. At a given twist and diameter (aka your bought barrel) you can get an idea of the maximum length of a still stabilized bullet.

The 0.922 bullet length in my sheet was taken from Quick Load (Version 3.6). Hornady's #45201 FTX 250gr bullet shows 0.922 length.
After reading your posting i took a FTX bullet and oh oh - it's 0.955/0.960 long ....

Second error in the data of QL (case length was/is 1.795 instead of 1.70 but the programer, Mr. Broemel, is informed and will change it in the next version). Ok, DID hornady change the length of the FTX?


Back to the greenhill formula: a nice little calculator is found at http://kwk.us/twist.html - he is using a corrected version of Greenhill. I used Greenhill because i knew i would not buy a 1:33 twist barrel and even if 1:36 is "better" i am bound to buy obtainable barrels, not theoretical better barrels.
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Re: BARREL LENGTH AND TWIST RATES DISCUSSION

Postby wildcatter » Mon Nov 26, 2012 9:02 pm

You guys are getting it.

Proper twist-rates are somewhat of a Voo-Doo, game.

I do not use Greenhill at all. It may or may not put you into the ball park and sometimes, using it, won't even get you into the ball park. I only use spin rates a.k.a. Revolutions. Even then the Rev's can put all over the board, but is far more accurate than Greenhill.

When doing a work-up for a new cartridge and Bullet combo. I accurately estimate the final FPS for that combo. You can do that a number of ways. I have even made a smooth-bore, shot the bullet, recorded the FPS and then choose the twist rate, according to the Rev's I need.

For instance, if I had chossen a 44 caliber bullet/cartridge combo, I'd take the middle of the road weight or the most popular weight, for that caliber and get the speed, of a known very accurate bullet in that caliber. If that bullet is most used in target matches, on say a Remington 44Mag cartridge, in say a 6" barrel, I ask myself,.."What are the Rev's of the most accurate loading, under the afford mentioned parameters? With those Rev's firmly established for the afore mentioned bullet and the other parameters, I then have to ask myself, given my increased FPS, in the new case, what is the twist rate that gives me the very same Rev's of the 6" 44mag accuracy loading?

This method still needs to be shot for effect and sometime the effects point out, that there is a compromise in the offing for the final twist rate. Hence, the VooDoo, but this is far and away better than Greenhill. Heck, with GreenHill, after 100+ years of development, we are still developing the 30-06..

..t.
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