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Barrel twist rate

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Barrel twist rate

Old 03-05-2003, 10:14 AM
  #1  
Nontypical Buck
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Default Barrel twist rate

can anyone tell me which bullets are better suited to different barrel rates? As in, are heavier bullets better in fast twist barrels or light bullets better in slower twist rates?

And what is consitered fast?
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Old 03-05-2003, 02:56 PM
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bigcountry
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Default RE: Barrel twist rate

Depends on the caliber. Like for a 30-06 and most of the calibers that use .308 bullet and with velocity over 2800fps, 1 in 10 twist is the norm. Bullet wieghts range from 150gr to even 240gr. But when you neck down the 30-06 to a .270 and shoot .277" bullets, and keep the 1 in 10 twist barrel, your optimum grains go from 100gr to 150gr. Reason being is when the bullet diameter gets smaller, and the length of the bullet would get longer for the same grains. And bullet length is a critical factory in bullet stabilization. So they drop the bullet wieght. Then neck down even further to 25-06, optimal goes down to 90gr to 120gr.

But then if you drop the velocity like in a .308Win, you have to loosen up the twist to 1 in 12 or 1 in 14 to shoot the 150gr to 200gr bullets.

So basically, they are all dependent on each other. Velocity, bullet length, bullet diameter.
 
Old 03-05-2003, 03:49 PM
  #3  
Nontypical Buck
 
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Default RE: Barrel twist rate

A rough approximation of the minimum twist rate can be firgured out by this formula:

Rate of twist= (150x(bullet diameter squared))/bullet length.

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Old 03-06-2003, 03:16 AM
  #4  
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Default RE: Barrel twist rate

the twist of a particular gun is set to accommodate the (typically) longest bullet commonly available in that caliber. The best two examples are the 270/7MM Mag and ther 6MM rem/.243 win....

The diameter difference between a .270 and a 7mm Mag is only .007 inch. That' s only the thickness of two human hairs....yet the 7MM bullets commonly include the 175 grain spitzers as opposed the 150 grain bullets for the .270. The 7mm rifles all have (roughly) 1 in 9 twists to handle the longer bullet and most all .270 rifles have 1 in 10 twist. The faster (1 in 9) twist is required to stabilize the longer bullet.

This does not mean that you can' t shoot shorter bullets in the 1 in 9 twist barrels.....you sure can!!! In the 7mm you could shoot a 120 grain bullet with a 1 in 14 twist, but you' d not like the results trying to shoot the 175 grain bullets in that same barrel.

Typically I say that with hunting rifles .264 and larger I prefer the faster twists because I' m likely to be shooting the longer bullets. However with varming bullets in .224 and .243 diameter, I prefer the slower twists because I often prefer to shoot the shorter bullets and at extreme velocities and the faster twists can cause such extreme RPMs that the bullet actually explodes from centrifugal force in mid air.....the bullets also tend to be thin jacketed as well.
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Old 03-06-2003, 09:46 AM
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Default RE: Barrel twist rate

I have a question. I have a 30-06 that is 1 in 10 twist. I have a .308 that is 1 in 12 twist. I notice some of the new .308s are 1 in 10 twist and they handle the heavier bullets better. Like my .308 shoots 150s well but the 30-06 shoots 165s better than 150. My guess is that the .308 would shoot 165s best if it was 1 in 10. Why even make a .308 1 in 12? They are only 200 fps max below the 30-06.
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Old 03-06-2003, 10:38 AM
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Default RE: Barrel twist rate

I think it' s because the 308 doesn' t have the case capacity to EFFICIENTLY shoot the heavier bullets... 180 and above. It CAN shoot them, but it' s just not quite as efficient from an over-the-counter hunting cartridge standpoint. An astute handloader that realizes that velocity isn' t quite the whole story can reload 180' s or 200' s in a 308 and have a great load that penetrates VERY well....... IF the twist in his barrel is fast enough to stabilize those longer than normal bullets.
The 30-06 has the powder capacity to propel those heavier bullets at " hunting" velocities, so it has a faster twist barrel to allow that. ...... Just my opinion....
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Old 03-06-2003, 11:05 AM
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Default RE: Barrel twist rate

Thanks! But why not just make all 308s 1 in 10 then?
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Old 03-06-2003, 12:06 PM
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bigcountry
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Default RE: Barrel twist rate

Due to the velocity drop of a .308 compared to a 30-06, You have to make the twist 1 in 12 to stabilize properly the bullets usually offered by the bullet manufactures 150gr to 220gr. Go to

www.stevespages.com

This guy has a downloadable bullet stabilizer calculator in excel. Play around with it. Velocity does play a part in this.
 
Old 03-06-2003, 12:25 PM
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Default RE: Barrel twist rate

Thanks! I guess with handloads, it all goes out the window. My uncle was shooting some .308 handloads with 150grain at 2850 in his .308. I was shooting some 30-06 loads for a little while with 150 grain at 2650... we ran them over a chronograph for verification.
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