Rate of twist to bullet weight classification

This is a discussion on Rate of twist to bullet weight classification within the Rifles forums, part of the Sniping Related category; Hey, guys. I have been reading some of the previous threads on rates of twist and various bullet weights. However, I am still unclear on ...

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  1. #1
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    Rate of twist to bullet weight classification

    Hey, guys. I have been reading some of the previous threads on rates of twist and various bullet weights. However, I am still unclear on this subject.

    The way I think I understand this concept is as follows: The heavier a bullet weight in grains, the faster the rate of twist in the rifle to stabilize the bullet to prevent excessive yaw. Thus, I would think that a twist rate of 1 in 10 would be correct for all .30 caliber bullets as anything faster would increase velocity to the point that the bullet "flies apart", for lack of a better phrase.

    However, between the threads I have read here and the firearms evaluations I have read in the gun press, I have noticed that there are different rates that correspond to the various bullet weights. For example, the 168 grain BTHP seems to prefer a 1 in 10 barrel, yet I have seen thread posts where SC members have fired the same bullet in their 1 in 12 Remingtons. Furthermore, I know that the military M24 and M40A3 SWS rifles utilize the 175 gr BTHP; the M24 being 1 in 11.25 and the M40A3 being a 1 in 12 as I recall. I can understand if the difference between the 11.25 and 12 twist is negligible to the point of being inconsequential.

    I am hoping that someone here can explain why the different bullet weights seem to correspond to the various twist rates when my limited understanding would seem to suggest that a 1 in 10 would be the optimum rate of rifling. Any help is greatly appreciated.

    Jim

  2. #2
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    It also has to do with the initial velocity of the projectile, which is why a .30 caliber .308 would have a different twist rate than a .30 caliber .300WM. Beyond that, and what you just mentioned, I do not know much on this topic. Perhaps one of our dedicated snipers will shed some more light on the subject?

    And I agree with your conjecture on the 1:10" twist being optimum theoretically, but for whatever reason the 168 and 175 grainer .308s seem to like a twist of about 1:12.

    Jake
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  3. #3
    Senior Member mlammers's Avatar
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    My 308s that I use for 175 and 168 have 1-10 twist if I build them. If I buy one, I am like you stuck with what the factory puts on it. Will 12 twist work yep, is 10 twist better yep, is every indidual rifle going to like a different round and velocity combination better yep. Personal opinion here, but if I were building a rifle for you in 308, and you wanted to shoot heavy(168-175) bullets I would put 10 twist barrel on it, unless you insisted on 12 twist(the customer is always right, even when I dont agree).
    ML313 Founder LOSOK Custom Arms LLC

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    Senior Member ekaphoto's Avatar
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    There is actually a formula some where that tells you the proper twist rate for the proper twist rate of a given bullet.
    John

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    Senior Member mlammers's Avatar
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    Easy way out is to call the ammo or bullet manufacturerm they have already done the homework for you, or even check on Midway USA, they will tell youin some of the tech tips on the bullets. Even then the individual rifle comes into play. I have a 243 here that should shoot 100gr bullets no prob, it is 10 twist, the ammo maker says that is enough. I shoot it, I get 3 MOA. I use 80 grain same bullet type and maker, I get .75 for 3, the first two were under .5. I choked the 3rd one, but I was using a 4X fixed scope at 100, so precision was not real easy.
    ML313 Founder LOSOK Custom Arms LLC

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    Quote Originally Posted by Jake S
    It also has to do with the initial velocity of the projectile
    The initial velocity of all my projectiles is zero :wink:

  8. #8
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    The Sierra 168 SMK was made for the 1:12 twist barrel. What I mean is that at the time this was the optimum bullet for the 1:12. I carried LC M852 (168 gr) rounds during the riot days, during civil disturbances in a Remington 700 HB varmint rifle (1:12 twist) while in the Army National Guard.

    Here is a chart:

    1:8" Generally used specifically with suppressed rifles can also be used to fire heavier bullets 220 gr. and up.
    1:10" for up to 220 gr.
    1:12" for up to 170 gr.
    1:14" for up to 168 gr.
    1:15" for up to 150 gr.

    Sometimes it just depends on the rifle. 1:10 and 1:12 seems to be the most popular and both will handle 168 grain bullets with no problem.

    Tom

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    What do you think about having "too much" twist in a .308? I've never heard of .308 bullets coming apart like light varmint bullets in a 1:7 .223 can, so wouldn't you want to select the twist for the longest bullet you ever wanted to shoot in that rifle?

    I know a 1:14 is sometimes recommended for the 155 Palma, and the guys running the 30 BR go for 1:17 or even 1:18 twist barrels. I wouldn't think shooting a 155 out of a 1:10 would somehow destroy accuracy, and I know the bullet wouldn't blow up, what do you think?

    1:10 seems like the best twist for a .308 to me, so you can shoot almost anything, and if you forsee using super-long match or extra heavy subsonic bullets you could do the 1:8 or even a 1:7.5 (Lost River 220 gr J40s) Thoughts?
    Bead Drawer

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    I think I have a very rudimentary understanding know. Let me see if I have this right, and please correct me if I am mistaken in my conclusion. Thansen's chart and one of the replies mentioned bullet weights being heavier and requiring more twist. The heavier bullets have more surface area for the same diameter, and are longer, as a result; consequently, they must spin faster to stabilize properly. Conversely, the lighter bullets have less surface area; therefore, a lower rate of twist is more appropriate. The Palma bullets are an example.

    Thansen, thank you for the chart. It did make things a bit more simple to understand. The key to the whole mess appears to be bullet size and weight. BTW, I never made it through high school physics. Might explain why my limited grasp of this concept. All responses are greatly appreciated. I may now proceed with an improved understanding of bullet dynamics.

    Thanks, guys.

    Jim

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