twist vs velocity


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xtarheel
April 26, 2012, 11:57 AM
Just wondering.. I have two AR's. One has a 1 in 12 twist the orher 1 in 8.

I understand the reason for the faster twist is to stabilize heaver bullets. But all things being equal, barrel length, etc. would the rate of twist have any effect on velosity?

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Robert
April 26, 2012, 12:15 PM
I could be wrong but velocity is a function of powder burn rate and barrel length. All the twist does is stabilize the bullet, it has no effect on velocity that I know of. But again I could be wrong.

chrome_austex
April 26, 2012, 01:14 PM
Bullets are pretty easy to spin down their long axis, so a faster twist won't eat much energy.

But ya, there is some loss there. It just may be less than the difference between two same-twist barrels from different manufacturers.

1858
April 26, 2012, 01:17 PM
there is some loss there. It just may be less than the difference between two same-twist barrels from different manufacturers.

I'd agree with this and add that any differences between the 1:8 and 1:12 barrels would be masked within the normal velocity variations for a given load in the same barrel.

TonyAngel
April 26, 2012, 03:54 PM
It's really nothing to expend any brain cells on.

Float Pilot
April 26, 2012, 04:53 PM
The twist rate accommodates the bullet's length and not the weight.

A tighter twist rate does add more resistance and increases chamber pressure.
When testing new FN made M-16A2s against older Colt M-16s, ( a 1 in 7 twist rate compared to the old 1 in 12 twist rate ), the tighter twist rate showed an average of 50 to 70 fps less velocity using both M-855 and M-193 ammo.

Though we used 5 rifles of each for our chronograph test, we still wondered if it may have been because the older Colt M-16s had very smooth barrels from extensive use as training rifles. While the FN Made M-16A2s were brand new.

In your case there will not be enough difference to matter.
If you were pushing very hot hand-loads, (like we do with the 6.8mm SPC) then you might want a looser twist rate.

Magoo
April 26, 2012, 05:21 PM
Bullets are pretty easy to spin down their long axis, so a faster twist won't eat much energy.

But ya, there is some loss there. It just may be less than the difference between two same-twist barrels from different manufacturers.

Nailed it.

Bullets are pretty easy to spin down their long axis, so a faster twist won't constitute a measurably significant percentage of the total energy of the system.

But ya, there is some loss there, there is more friction between the bullet a barrel so more work has to be done to move it through there. It just may be less than the difference between two same-twist barrels from different manufacturers, or variables presented in the field.

But I might be wrong :o

rcmodel
April 26, 2012, 05:28 PM
Considering that the bullet has the rifling engraved on it and is up to nearly full rotational speed by the time it has moved one bullet lengh into the rifling?

Not much.

rc

jmr40
April 26, 2012, 06:15 PM
the tighter twist rate showed an average of 50 to 70 fps less velocity using both M-855 and M-193 ammo.



The different twist may well have accounted for the difference. Especially since it was consistent with several rifles, but 20-50 fps difference is common between various rifles shooting the same ammo through equal length barrels with the same twist. I've seen well over 100 fps difference between the fastest rifle and the slowest I've tested.

I'd really never thought about it, but shooting slightly slower through a faster twist does make sense. But I believe it is just one of several factors that effect velocity and the difference is probably not enough to worry about.

Samclrk
April 26, 2012, 06:37 PM
Then just make ALL barrels with a two or three twist and not worry with which is best a ten or eight twist..

allaroundhunter
April 26, 2012, 06:39 PM
Then just make ALL barrels with a two or three twist and not worry with which is best a ten or eight twist..

Um, what?

Samclrk
April 26, 2012, 06:43 PM
Why then did Winchester and Remington and every body else build fourteen,sixteen twelve,and ten twist barrells ..Why not just build two or three twists and be done.

allaroundhunter
April 26, 2012, 06:47 PM
Why then did Winchester and Remington and every body else build fourteen,sixteen twelve,and ten twist barrells ..Why not just build two or three twists and be done.

Because in theory, each different bullet weight will shoot best out of a specific rifling twist. For example, the optimum twist for a 55 gr is not the optimum twist for a 69 gr when maximum accuracy is desired.

Samclrk
April 26, 2012, 06:52 PM
Thank you ,Sir.

Owen Sparks
April 26, 2012, 06:52 PM
All things being equal a tighter twist does increase pressure. The twist rate for the 6.8 SPC II barrels has been slowed to allow to burn more powder and achieve a faster velocity.

68wj
April 27, 2012, 01:00 AM
Since I saw 6.8 SPC mentioned a few times here, I thought this would be relevant to the conversation. Perhaps second only to .223/5.56, the 6.8 has been the subject of much debate regarding twist and its affect on velocity and pressure. The following was generated by a 6.8 barrel maker currently producing 1/7 twist barrels to stabilize 200 gr .277 bullets, but still reports to function well with the commonly found lighter loadings (he is also a member here and might be able to add to the discussion). This assumes that all is equal between barrels except the twist rate.

http://img.photobucket.com/albums/v665/hautlipz/velocitydependenceontwist-1.jpg

http://img.photobucket.com/albums/v665/hautlipz/pressuredependenceontwist.jpg

I was able to use one of these 1/7 barrels recently with 100 grain handloads that I had worked up for a 1/11 barrel. No chronograph was present, but on paper at 100 yards, and steel at 200, I didn't see anything wrong and the brass looked fine.

michael5446
April 27, 2012, 04:06 PM
simple physics, just depends how you frame it... it all has to to do with the coefficient of friction, the more more resistance you apply(ie more twist) the more back pressure and lower speeds....

hope that helps

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