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Old December 17, 2012, 10:10 AM   #1
cannonfire
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School me on AR15 twist rate

So I'm just learning about the benefits of each twist rate and what weight bullet will be best for each twist rate.

From what I've learned, the slower the twist rate (1:7) the better it will stabilize heavier bullets. The faster the twist rate (1:9) the better it will stabalize the lower weight rounds.

What about 1:8 twist? What are the benefits of a 1:8 twist compared to the others?

Thanks for any info
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Old December 17, 2012, 10:16 AM   #2
Woody55
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The 1:8 is a compromise between 1:7 and 1:9. Keep in mind that what you gain in making the bullet spin faster, you are losing in forward speed. There is only so much energy in the propellant and if more of it is going into the rotational energy of the bullet, there is less to go into the translation energy.

The original M16 and M16A1 rifles were - if memory serves - !:12. That resulted in a 55 grain bullet going around 3200 feet per second with military rifle and ammon.
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Old December 17, 2012, 10:30 AM   #3
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First, 1:9 twist rate is a slower twist rate, 1:8 is a faster twist rate and 1:7 is the fastest (current) twist rate (I believe).

Second, a faster twist rate is desireable and even necessary to stabilize the heavier grain bullets that the 5.56/.223 are now available with.

I've read a quick and dirty breakdown that might help and it is by no means a hard and fast chart to go by.

1:9 = 40gr - 62gr
1:8 = 50gr - 70gr
1:7 = 60gr - 77gr+ (77gr bullets being the heaviest available if memory serves)

You can still shoot any 5.56/.223 out of whatever twist rate barrel you have, it just might not be the optimal twist rate for the bullet weight you are shooting and therefore can affect accuracy, especially at longer range.
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Old December 17, 2012, 10:40 AM   #4
kraigwy
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A rule of thumb is:

The Faster Twist will stabalize light bullets, the slower twist will not stabalize heavy bullets.

Meaning a 1:7 can shoot 80-90 gr bullets, but it will also work with 50 gr bullets.

A 1:12 will stabalize the 50-55 gr bullets but will not stabalize the heavier bullets.

For an example, the 5.56 Mann device (used for testing military ammo) is a 1:7 twist. The ammo used to check the Mann is 52 gr Match bullets.

The below chart shows the army shooting 300 yard targets with heavy bullets in a 1:12 and light bullets in a 1:7.

If you have a choice, go the fasted twist you can. You can shoot 50s to 90 gr bullets.

Many say that the fast twist will over spin lighter bullet causing them to come a part, this may be true if you use light 40 gr pills designed for the Hornet, but any good quality 52-90 gr bullet will work with the faster twist.

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Old December 17, 2012, 11:56 AM   #5
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Quote:
The original M16 and M16A1 rifles were - if memory serves - !:12. That resulted in a 55 grain bullet going around 3200 feet per second with military rifle and ammon.
The earliest ARs, pre-M16, were 1:14. They didn't even stabilize 55s all that well, but it contributed to the "explosive" wounding reported by Army advisors in Vietnam in the early '60s.
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Old December 17, 2012, 12:50 PM   #6
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Quote:
The 1:8 is a compromise between 1:7 and 1:9. Keep in mind that what you gain in making the bullet spin faster, you are losing in forward speed. There is only so much energy in the propellant and if more of it is going into the rotational energy of the bullet, there is less to go into the translation energy.
I'd always been curious about this idea, so I did some informal testing a few summers back using my handloads and 20" AR uppers with different twists. Though the physics of it makes me believe I should see a loss in velocity with the faster twist, the chronograph showed no appreciable differences between the different twists. Perhaps the differences would have been more pronounced in a larger caliber, shorter barrel length, etc; like is said, it was informal testing and by no means comprehensive. But what results I did get leads me to wonder how much velocity difference twist really makes overall. Perhaps someone else can add to the subject, becuase it does have me curious.

Regarding the OP's question, it's worth mentioning that there are other factors in bullet stabilization besides twist. Length of barrel, velocity of round, and shape of bullet all influence stabilization as well. So as a for example, I've found that the Hornady 75 BTHP will stabilize just fine and shoot quite accurately in my 1:9 twist barrels, but that the Hornady 75 Amax will not. The bullets weigh the same, but when you compare the two you see that they are shaped differently so that one will engage the rifling and stabilize and the other will not in a 1:9. I've also been able to get some of the lighter weight bullets to stabilize in faster twist barrels by backing my loads down so they're not going as quickly, and found at times I can push heavier bullets a little faster and get them to stabilize in a twist that might be too slow.

So point is, while twist is the primary influence on bullet stabilization, there are other factors at play as well. Generally speaking though. I'd agree with Kraig that you're best to get the faster twist barrels unless you are looking for something more specifically to shoot lightweight varmint style bullets.
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Old December 17, 2012, 02:08 PM   #7
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Thank you everyone for the info.

Quote:
First, 1:9 twist rate is a slower twist rate, 1:8 is a faster twist rate and 1:7 is the fastest (current) twist rate
You're right. I don't know why I mixed them up when I posted the question. I knew that 1:7 was faster than 1:9, but thank you for bringing it to my attention.


Kraigwy,
As usual you are a wealth of knowledge. Thank you for help.

Quote:
Meaning a 1:7 can shoot 80-90 gr bullets, but it will also work with 50 gr bullets.

A 1:12 will stabalize the 50-55 gr bullets but will not stabalize the heavier bullets.
From your post I understand that a 1:7 (or the fastest twist rate) will give you a wider variety of bullet weights that you can use over slower twists. But would a 1:12 twist be better for say a 55gr 5.56 than a 1:7 in terms of (all else being equal) groupings and accuracy? You just wont get the same performance from a 80-90gr compared to using the 1:7. It seems (and I'm learning here) that if I wanted to strictly target shoot with 55gr, I would be better off with a 1:12 rather than having a more versatile option with the 1:7.

^I hope that makes sense.

And for what its worth this is only for my knowledge, I'm not looking into buying another AR just yet. But on mine I have a BCM 16" mid with a 1:8 twist. I don't target shoot and I use 55-62gr ammo.

Thanks guys
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Old December 17, 2012, 02:13 PM   #8
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scsov509,

Thank you for pointing out that their is more that just twist rate when involved with accuracy/stabilization. I'm relatively new to the more precise aspects of the inner working of firearms and target shooting so as I'm aware that there is more to it than bullet grain and twist rate, I'm trying to tackle one thing at a time hahaha.

The marines taught me to shoot, but they didn't teach about twist rates, bullet weight, and bullet design. Thank you for your help
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Old December 17, 2012, 03:22 PM   #9
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When determining which kind of bullet benefits most from which rate of twist, the length of the bullet should figure into the equation. As scsov509 pointed out, the configuration and shape of a given bullet has as much to do with stabilization as its weight does. When in doubt, I would opt for the quicker rate of twist.
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Old December 17, 2012, 03:26 PM   #10
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I have shot bullets weighing 68 grains in 1/9 twist rifles.
The combination works fine.

I don't think 1/7 , 1/8, or 1/9 matters unless you are shooting

in sanctioned competition or using very heavy bullets.
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Old December 17, 2012, 04:06 PM   #11
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...and single loading as well. Those long 77 grainers won't even fit in a magazine anyway.
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Old December 17, 2012, 05:49 PM   #12
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Quote:
From your post I understand that a 1:7 (or the fastest twist rate) will give you a wider variety of bullet weights that you can use over slower twists. But would a 1:12 twist be better for say a 55gr 5.56 than a 1:7 in terms of (all else being equal) groupings and accuracy?
I have read that someone makes a 1:6.5 twist but I don't know anything about them.

As too the 1:12 shooting 50-55 gr bullets better then 1:7, I'm not sure that is true, After shooting light bullets (Mainly 52-53 gr match bullets) I find my 1:7 shoots them as well if not better then my 1;12 223s.

The best bullet I've seen out of the Mann device was the 52 gr SMK, pushed by 25 gr. of 3031 and Remington brass.

If the 1:6.5 twist exist, I don't see the advantage, I shoot 90s out of my 1:7 quite well. The problem with the 80s and 90s is they don't fit in the magazine as they have to be loaded longer. Which is ok in match shooting because you single load anyway.

Understand its not really the weight of the bullet, but the length. Velocity helps spend the bullet faster, that we know. But in long range shooting, 600-1000 yards or more, you dont get the velocity you need, that is where the faster twist comes in.

You could probably get by with a 1:9 out of a 22-250 or 220 swift because of the velocity, but you can't get that kind of velocity in a 16-20 inch AR.
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Old December 17, 2012, 06:12 PM   #13
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1:7, 1:8 and 1:9 will work with most 5.56/.223 ammo the average person will use. I would read carefully what kraigwy wrote before choosing a twist rate.

For most of us who won't be going much under 55gr ammo, the 1:7 twist makes a lot of sense. I have three ARs with 1:7 and one in 1:9 and really can't see much difference with 52gr ammo (I haven't tried anything lighter). There definitely can be a disadvantage with heavier ammo and 1:9.
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Old December 17, 2012, 06:50 PM   #14
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@RickB, Thanks for the correction on the twist of the M16 and M16A1.

@scsov509, The trend as the rate of twist is going to be as you say. The magnitude is another question. Assuming my freshman physics and integral calculus is right, adding a typical twist makes a small negative correction to the velocity that is proportional to the twist squared times the radius of the bullet squared. Since the number of turns per inch of the twist and the radius of the bullet in inches are pretty small numbers, your meter might not be able to detect the change.

In which case what I said about velocity is correct but kind of irrelevant.
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Old December 17, 2012, 10:39 PM   #15
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Quote:
@scsov509, The trend as the rate of twist is going to be as you say. The magnitude is another question. Assuming my freshman physics and integral calculus is right, adding a typical twist makes a small negative correction to the velocity that is proportional to the twist squared times the radius of the bullet squared. Since the number of turns per inch of the twist and the radius of the bullet in inches are pretty small numbers, your meter might not be able to detect the change.

In which case what I said about velocity is correct but kind of irrelevant.
Interesting. Given the math you propose, I wonder if you'd be able to see a more substantial difference comparing twists in a larger caliber where you've got a larger diameter? The physics and math make total sense, I'm just curious to see if it actually translates into an appreciable difference in a rifle.

Quote:
It seems (and I'm learning here) that if I wanted to strictly target shoot with 55gr, I would be better off with a 1:12 rather than having a more versatile option with the 1:7.
I personally would never go slower than 1:9 in a 223 barrel unless you were planning to shoot a lot of lightweight varmint bullets. The 1:9's will shoot a pretty wide range of bullets quite well, whereas the 1:12 or 1:14's limit you to smaller bullets. So I think that a 1:9 is the best all around twist if you want the widest range of options. But if you don't ever plan on shooting light, thinly jacketed varmint bullets then a 1:8 or 1:7 is even better since you can shoot all but those varmint bullets fairly well.
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Old December 19, 2012, 11:40 AM   #16
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Thank You everyone for your responses. The info you guys posted are helping me further my learning of stabilization and what goes into it (even if I havent grasped all the concepts yet).

Sorry it took so long to respond... Studying for finals... you know how that goes
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Old December 19, 2012, 11:42 AM   #17
scsov509
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Quote:
Sorry it took so long to respond... Studying for finals... you know how that goes
Isn't twist rate and bullet stability on your final exam?
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Old December 19, 2012, 11:51 AM   #18
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Seems to me given the recent scarcity of affordable ammo, this is particularly relevant. For example, I wanna get the best deal for my 6920 (1:7) for range practice while using proper ammo. I've been mainly watching 62gr (e.g. M855) but good to know I can use lighter and heavier.
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Old December 19, 2012, 12:09 PM   #19
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Now that I think about it, I really DON'T understand twist mechanics...

Regardless of the bullet proper's weight, length, or internal composition, the barrel determines how fast a bullet twists in flight. How does its weight affect how the copper or outer layers break up from spinning?
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Old December 19, 2012, 01:10 PM   #20
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Quote:
Isn't twist rate and bullet stability on your final exam?
I WISH! Then that would mean that I would be studying a subject I enjoy!
Instead its how Buddhism traveled from India to Japan. Fun!

Again thanks
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Old December 19, 2012, 01:53 PM   #21
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@AndyWest,

Quote:
Regardless of the bullet proper's weight, length, or internal composition, the barrel determines how fast a bullet twists in flight. How does its weight affect how the copper or outer layers break up from spinning?
The barrel determines how far the bullet has to travel in order to spin around once. The forward speed of the bullet determines how much time it takes to travel that distance.

The faster the forward speed, the less time it takes to spin once or the faster it is spinning.

The faster something spins, the larger the force that acts radially out from the axis of the barrel (or the axis of the spin)(the centrifugal force if memory serves). It's that force that tends to make the bullet come apart.

So, to answer your question about weight, the less the bullet weighs the faster it is going to move forward (assuming the powder charge stays the same), the faster it spins and the more the centrifugal force is.

The main point is that it isn't how far the bullet has to travel to spin a certain angle that determines the centrifugal force, it's how long it takes to spin through that angle.

As to whether the bullet comes apart or not under this force, that has more to do with how it is constructed.
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Old December 19, 2012, 02:14 PM   #22
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Ah! That's what I was missing from my mental picture. The barrel determines how many TIMES any-weight bullet spins over a given distance but the bullet's velocity determines how many in a given time. Hence the centrifugal force. Thanks, it's so clear now that I feel stupid having asked that.
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Old December 19, 2012, 02:29 PM   #23
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Ok, so all the main topics have been touched on, but I feel some are missing.....

First, over stabilization issues are less so therefore you should error on the side of overstabilization. Under-stable bullets will keyhole everytime. Being way over stabilized like 40 gr FB in a 1:7 ar will likely just add .25-.5" to your group size...under stable like 77's in a 1:10" will likely keyhole at 25 yards and be off paper at 100!

Second is modern history. At some point we went to 1:9 for 62's....then to 1:7 for 75gr mag length rounds. I think the 75 gr ammo has not seen wide spread use.

Third, 1:9 - 1:7 will all do the most popular AR ammo with 55 and 62 bullets well enough. I load 68's mag length in my 1:9, but will not reach to 75's...no need. For 0-600 yds, the 68 bc is fine for me.

So for mag length surplus ammo, 1:9 is likely your best bet. If you see 75's or 90's in your future, adjust accordingly.
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