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30-05-2020, 23:09   #1
badshot
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twist rates

buying a 223
whats the difference between a 1/12 twist and a1/8 twist
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30-05-2020, 23:49   #2
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12" inch twist will stabilise the shorter bullets. Typically up to 55gr. 8" twist can stabilise those longer ones (tend to be heavier) 69g, 77g etc. I used to shoot some 69g SMK Fiocchi in my Tikka T3 8" twist and they were great.

Most ammo on the shelf is 55g and less. You sometimes see 64g and Lakelands has a buncb of 69g.

If you're only going to shoot the lighter faster stuff, 12" is more than sufficient. If you think you might want to shoot the heavier (read longer) bullets, then go for the 8" or faster twist.

My current 223 is a 9" twist. Shoots pretty much everything up to 69g really well. Anything above that though, tends not to stabilise too well.

Last edited by alanmc; 30-05-2020 at 23:54.
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31-05-2020, 00:47   #3
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I am in the exact same position. Decided a couple of weeks ago to buy a .223 and like you need to understand twist rates.

After reading many articles and advice from many people. This is what I have come up with.

1:12 twist, will shoot 55gr rounds really good, up to 69gr, some rifle have trouble with 69g, but 63gr not issue.

A 1:12 will also shoot lighter rounds, 50gr, some lighter loads, but you don't seem too many here in Ireland.

Now, 1:8 twist will shoot 55gr no problem, and with a good round will shoot as good as the 1:12. Where the fast twist comes into play is with heavier loads. The 1:8 will shoot rounds up into the 80s.

So the fast twist is more flexible, but some rifles will not shoot round below 55gr.

I have a Tikka T3x .223 1:8 twist ordered. (Some people have said this rifle will even shoot 50gr)
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31-05-2020, 08:28   #4
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Really good advice. My understanding is that the length of the barrel is also part of the equation for working out the best option (i.e. 1/8 in a 20in is a different beast than 1/8 in a 28in. Maybe someone can explain in more detail.

I remember seeing some sort of formula a few years ago for working out ammo v twist v length etc..
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31-05-2020, 14:01   #5
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When discussing twist rates and suitable ammo choice we talk of bullet weight, but the weight of a bullet is somewhat misleading. What we actually need to know is the bearing surface length.

Every calibre is fixed. Meaning a 40 gr bullet and 75 gr bullet in 223 have the same diameter. To make it heavier they make it longer. This is when twist rate comes into play. The longer a bullet the more bearing surface that engages the lands and grooves. The bearing surface of each bullet can vary with some "lighter bullets" having a longer bearing surface than other bullets in the same weight category. This means if you have (made up example) a 55 gr Hornady and 55gr Federal bullet well both can weigh the same but the Hornady may have a shorter bearing surface than the Federal so it'll act like a lighter bullet and work in a slower twist rate than its recommended one.

This picture (excuse the microsoft paint drawing) the difference between a 40gr bullet and 75 gr bullet.




However if i were to talk to someone about suitable bullet for twist rate in terms of bearing surface i'd be talking in thousandths of an inch. For example (and excuse the numbers they are completely made up because i don't have the actual numbers) a 40 gr would not be a 40 gr but a 0.4721 bullet compared to the 0.5512 bullet (75 gr).

It's easier to talk in weights.


The reason i say weight is not ideal is because sometimes this weight to bearing surface is not always as predictable as you'd think. I'll discuss this in 308 as i know them. My rifle is 1:10 twist. It's suitable for bullets of 168 - 185 gr. They have longer bearing surfaces than 123 - 155 gr bullets so work better in the faster twist rate.

However the Lapua Scenar 155 gr is a long bullet. So long in fact that it's bearing surface length is the same as the 175 gr Sierra, and close to the 180 gr bullet. Because of this the bearing surface is longer, and the "lighter" bullet acts more like a "heavy" bullet. This means that although it's a 155 gr bullet it works, quite well, in a 1:10 twist.

It's for this reason that some rifles will shoot heavy or light bullets that seem to contradict the twist rate:bullet weight ratio. This is because of bearing surface length. Example of this oddity below:

Attached Images
File Type: png Bearing Surface.png (19.5 KB, 188 views)
File Type: png Bearing Surface 1.png (19.5 KB, 189 views)
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31-05-2020, 22:29   #6
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Originally Posted by freddieot View Post
Really good advice. My understanding is that the length of the barrel is also part of the equation for working out the best option (i.e. 1/8 in a 20in is a different beast than 1/8 in a 28in. Maybe someone can explain in more detail.

I remember seeing some sort of formula a few years ago for working out ammo v twist v length etc..
Greenhill formula, when I googled it to check, I came across the Miller Twist Rule

https://en.wikipedia.org/wiki/Miller_twist_rule
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31-05-2020, 22:36   #7
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Out of curiosity for a .223, why bother making a 1:12 twist barrel at all? Why don't all manufacturers make all their rifles in 1:8?
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31-05-2020, 22:44   #8
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Because the most commonly used ammo seems to be in the 40 to 55gr mark which is ideally suited to a 1:12 twist rate.

1:8 twist rate is only suitable for ammo in the 69gr plus range.
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31-05-2020, 22:44   #9
zeissman
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Out of curiosity for a .223, why bother making a 1:12 twist barrel at all? Why don't all manufacturers make all their rifles in 1:8?
If you want to shoot the lighter bullets a 1/12 twist is the best choice..
Some 1/8 twist rifles will shoot the lighter bullets but some wont.
My mate has a tikka that is very accurate with the heavier bullets but not so with the lighter stuff.
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31-05-2020, 22:47   #10
Feisar
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Out of curiosity for a .223, why bother making a 1:12 twist barrel at all? Why don't all manufacturers make all their rifles in 1:8?
Apparently a 1:8 gives up some velocity with the lighter bullets. A 1:12 is optimum for varminting as it maximizes a 40 to 55 grain bullets velocity.

Edit - I'm quoting textbook knowledge, I haven't shot the same rounds side by side in a 1:8 vs 1:12
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31-05-2020, 23:51   #11
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Its also about stability in flight. The wrong bullet with the wrong twist rate rifle will cause instability in flight and result in loss of accuracy or keyholing.
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01-06-2020, 00:23   #12
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I find the Berger Twist Rate Calculator really useful. It has presets for Berger bullets obviously, but you can get specs from other bullet manufacturers (Sierra, Hornady, Barnes, Lapua, etc) from their websites and input parameters (length, Ballistic Coefficient) along with the twist rate of your barrel to produce a natty graph which tells you how stable your combination is.
I wouldn't use it as gospel - it's not a replacement for actually trying a bullet in your rifle, but it's a good guide.

Last edited by alanmc; 01-06-2020 at 00:27.
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01-06-2020, 00:43   #13
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Quote:
Originally Posted by TheEngineer1 View Post
Out of curiosity for a .223, why bother making a 1:12 twist barrel at all? Why don't all manufacturers make all their rifles in 1:8?
I've kinda wondered this myself. Mainly when I'm trying to figure out why my .308 barrel is a 12" and not a 10" twist. If you google enough, you'll find holy wars about "over stabilisation". There are "stories" and myths about bullets flying apart due to being over spun. Many refute this and yet many swear it's true. I remain unconvinced.
There are so many variables in play here, not just twist rate (velocity - a function of barrel length, charge weight and powder burn rate, bullet shape - as Cass has explained above, barrel construction and rifling type - button vs cut vs polygonal).

All of these variables contribute to the revolutions per second on the bullet when it leaves the barrel. IMO, there is no substitute to trying bullets in your rifle. The weight vs twist is a good guide, but not always the truth. For instance, my 12" twist .308 barrel on paper shouldn't stabilise a 175g TMK bullet .... but it does.
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01-06-2020, 09:46   #14
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Really good advice. My understanding is that the length of the barrel is also part of the equation for working out the best option (i.e. 1/8 in a 20in is a different beast than 1/8 in a 28in. Maybe someone can explain in more detail.
The rate is the same, so the rotational force is the same. But the longer barrel has that force acting on it for longer, so generates a more spin.

1:8 in a 20" barrel is 2.5 revolutions.
1:10 in a 25" barrel is 2.5 revolutions.
1:12 in a 30" barrel is 2.5 revolutions.

All thing being equal, the three barells above would produce the same amount of muzzle spin, and this stabilize to the same degree. It's actually the number of revolutions that is relevant rather than the twist rate.
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01-06-2020, 11:17   #15
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The rate is the same, so the rotational force is the same. But the longer barrel has that force acting on it for longer, so generates a more spin.

1:8 in a 20" barrel is 2.5 revolutions.
1:10 in a 25" barrel is 2.5 revolutions.
1:12 in a 30" barrel is 2.5 revolutions.

All thing being equal, the three barells above would produce the same amount of muzzle spin, and this stabilize to the same degree. It's actually the number of revolutions that is relevant rather than the twist rate.
Nothing to do with barrel length. 1in12 means the bullet rotates once in every 12". This equates to, when the bullet leaves the muzzel and travels 12" it has rotated once.
A 1in 8 has rotated once in 8" therefore it has more spin, it has more giroscopic effect and more stability
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