I'm getting back into this project, and need a barrel for my AR-308. It seems like splitting hairs with these twists though. I would like to use 140 grain bullets, and will reload.

First rifles Remington chambered in it were for 140gr bullets and had a 1:9.25 twist, have one in the safe. What barrel length are you planning on running?

If buying a barrel for one today I would go 1:9 or what ever the Berger twist rate calculator shows would run 162gr bullets. Just making sure to have other options in bullet weight.

1:9 7mm-08 will run 140's up to 168's.

9 twist will do it all. Depending on speed and bullet choice one could get by with a 10 but it's not ideal imo.

I'm 9.5 on ours and they handle everything we shoot

Since an AR already loses 100 fps compared to a bolt gun, I'd go faster twist than what you need. Most 140 grain will work fine in a 1:10" twist. But it wouldn't work well for the 162-168 grain bullets as good. It would handicap the rifle some. I would lean heavily to a 1:9" twist. Plus the faster twist helps with better terminal performance.

I went 1:11" twist on my 300 WM since it was "just enough". I now wish I had gone with a 1:10" to fully stabilize the heavy 30 cal bullets.

Chad,

why do you say a 1:9” will have better terminal performance than 1:10”?

I think Chad is referring to RPS on the bullet, the faster it is spinning the more centrifugal force is exerted making it open a little faster giving a larger frontal diameter a little quicker and separating out those secondary projectiles John Nosler loved for secondary wound cavities. With barnes bullets it helps initiate expansion a little quicker.

Just to make math easy, say a deer is 18” thick.

1:9” will make two cimplete revs passing through. 1:10” will make 1.8 revs.

Does not seem like a difference of significance assuming all other variables are the same.

I went through a discussion here recently on the my recent 7mm08 project with a 1:10 twist.

Results: 17" barrel is shooting 140s and 120s very well.
140 averages 2800 FPS
120 is 2900 FPS

I am digging this little bolt action, too it out yesterday evening and center punched a doe through the top of the head from a tower blind with the 139 GMX.

BTW, I have quite a bit of factory ammo available I will sell off if you want to test some.

Hey Jeff, basically, what kmon explained. When a bullet is spun faster (faster twist), the bullet has more rotational force. When the bullet enters soft tissue and the bullet is upset (expands), it will have more expansion and more of a chance of tumbling and causing more damage. It's the same principal why the military went to a 1:7" twist in the M4's, from the previously slower rifle twists of 1:9" and 1:12" twists. Since NATO regulates the type of bullets the military can use (FMJ, etc), the bullets are difficult to expand. So when you spin them faster with more rotational force, an FMJ bullet will tumble in soft tissue and cause massive damage. It kind of does the same thing with hunting bullets, and allows for the bullet to expand and possibly fragment more.

For example, I had a customer shoot a large buck in the neck last weekend with a 1:8" twist 28 Nosler and 195 grain EOL bullet from about 140 yards. The bullet went in, and never came out. The rotational force on this bullet at that close of range caused massive fragmentation.

Chad,

the difference is 2 revs (or less) vs. 1.8 (or less) revs passing through the deer.

Hard to see how that 0.2 extra rev could be a difference of significance.

Jeff,

You're leaving out the revolutions the bulley made on the way there.

Not following you? Please explain.

The bullet makes one rev in 9" of linear travel or 10" of linear travel.

If a deer is 18" thick (most are less, just picking that to keep math simple), with 1:9" twist, the bullet makes one complete rev over 9" of distance. If the deer is 18" thick, the bullet will make two complete revolutions passing through. With 1:10", the bullet will make 1.8 revolutions passing through.

Jeff, it's not the number of revs, it's the bullet expansion that results from the faster revs. Don't think about 1.8 vs. 2 revs, think about the bullet expanding more and causing more damage. P_102

Guess I am not understanding how a difference of significance of 1.8 revolutions vs. 2 revolutions going though the deer's body can make any difference of significance.

The bullet does not stop and turn high rpm's like a chainsaw.

You guys are not understanding basic physics.
"The bullet does not stop and turn high rpm's like a chainsaw."

The revolutions in a barrel do not correlate to a direct 1 in 9" twist inside a cavity. You need to be thinking in RPMS like a drill. Remember that once that bullet contacts resistance (flesh), the RPM's will decrease. That bullet will still spin a heck of a lot more than two revolutions within the cavity.

See this article:

http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

Read it and the article says 1:12" is one rev per 12" of travel.

Twist rate is a measurement of revs per unit of distance traveled.

RPM's is a measurement of revs per unit of time.

1:10" twist is 1 rev per 10" of linear distance by definition.

Whether it takes 1 millisecond or one month for one rev is a function of velocity, not distance traveled.

Perhaps you can quote the sentence that says the bullet slows down and increases rpm's per inch of travel because I am not seeing it in that article.

once the bullet hits, all bets are off

Tend to agree with that. Bullet construction, design, and what it hits matter way more for performance than the twist rate. Just my observation from the animals I've shot using 1:9", 1:10", 1:11.25" and 1:12" twists.

Jeff, it's simply centrifugal force. A bullet spinning faster has more internal force around its axis trying to break it apart, causing it to expand more (or even separate) when it hits the deer. A few days ago there was a post about a bullet spinning too fast and losing its jacket before reaching the target, this is a good example. P_102

RPM's of a bullet is muzzle velocity times 720, divided by rate of twist. RPM= MV x 720 / rate of twist

If you take a typical 7-08 with 140 grain at 2800 fps, here's the numbers.

A 1:9" twist has 224,000 RPM's. A 1:10" twist has 201,600 RPM's. That's an 11% increase in RPM's, which will be even higher % in rotational force. (I don't remember the formulas for that). This rotational force will help in expansion and terminal performance.

But given the pieces of a millisecond we're talking about in the few inches a bullet travels in a deer, I think you are overvaluing the 11% difference.

And If I blew up a 195grn bullet on a deer's neck with no exit, I'd never hunt with that bullet again.

I knew this dialogue would go down this path when I asked the question initially, and knew there would be resistance, but thought it worth beginning to shed some light on a very common misunderstanding. Believe me, I understand how easy it is conceptually to confuse and conflate the two completely different measurements. I did not wake up to the difference until just this year.

I also used to think that the bullet hit a deer doing 200,000 RPM is like a chainsaw turning 200,000 rpm's hitting the deer, and a faster twist rate meant the bullet is spinning 10's of 1,000's of times inside the deer - it is not.

The twist rate literally describes the distance the bullet travels in one revolution. 1 in 12" means the bullet turns one time in a 12" distance - changes in velocity will change rpm's, but do not change that distance traveled per rev. Changes in velocity will change how long it takes the bullet to travel that distance, but it still will be making 1 rev per 12" of distance traveled, or 1 rev in 9" or 10" whatever the twist rate may be.

Once the bullet leaves the barrel it maintains that same rate of rev vs. distance independent of velocity. RPM's do not measure the distance traveled per rev. A 1:12" twist means the bullet will make 5,280 revolutions traveling 1 mile, which is 5,280 feet. Higher velocity will increase rpm's, but it will not change the number of revs made in the 1 mile distance. A bullet fired from a 1:10" will make 6336 revs in 1 mile regardless of velocity will effects rpm's.

Changes in velocity can change the time it takes to cover that distance and will change the number of revs in a minute, but will not change the number of revolutions over that distance.

Jeff,

You are still failing to grasp the physics and the math. The twist rate causes the spin. After that, RPM become a function of velocity. Velocity over distance traveled is not a constant. The RPM slow as the velocity slow, as does the distance traveled. Simply way to look at is, are the initial twist rate, velocity, and atmospheric pressure conducive to stabilizing a given bullet over a given distance? Think of throwing a football. And some point the decreasing rotation/gravity/atmospheric pressure cause the football to become unstable and wobble. Same thing happens to a bullet. The football does not maintain a constant velocity or spin, neither does the bullet. Ie. the closer the receiver is to were the football is thrown, the more spin the football will have, the more stable it will be, and the faster it will be traveling.