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Thread: Twist for 308 WIN???

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    Twist for 308 WIN???

    It seems most guys run the 175 grain bullets in the 308 Win.

    Why are most people running a 1/10- 1/11 twist???

    If a 1/13 will work

    A 1/12 or 1/11.25 seems like a better choice. I have always heard it is better to over stabilize than under stabilize. It seems though that shooters should run a optimium twist rate. If this does not hold true we would all shoot with the fastest twist rate avaliable.

    I am about to have a 308 WIN built. I do not plan on going heavier than a 175. The 175 is my first choice and the 155 my second.

    Your comments are welcome>

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    Re: Twist for 308 WIN???

    10 twist will work with any bullet the .308 will shoot.

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    Re: Twist for 308 WIN???

    My recommendation is to err on the side of the faster twist rate, with 1 in 11.25 or so considered optimum. 1 in 10 is also very, very acceptable. As was previously mentioned, if you get a high hard one for a heavier bullet like the 208 AMax, you'll be glad you opted for the higher twist rate. Remember, spin rate is your stabilizing factor, and you want the bullet as stable as possible if you're shooting the far end of the cartridge's capabilities. If I were putting a 308 together now, I'd choose the 1 in 10 twist and 24 to 26" barrel. Like you, I think the 175 SMKs are the bullet to shoot.

    Good Luck,

    Wes
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    Re: Twist for 308 WIN???

    I have a 12 twist in a 23" barrel. 168's and 155's work great. for some reason 175's won't group well at all.
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    Re: Twist for 308 WIN???

    Try running the 175s above 2675fps. That's what I had to do with my .308 when it had a 12 twist barrel a few barrels ago. below 2675fps and the groups opened up but above it they got acceptable. A stout charge of Varget will do it. Since then I only put on 10 twists so that isn't a problem. I run the 178 AMAX now.

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    Re: Twist for 308 WIN???

    See Don Miller's stability formula here:

    http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi

    Ideally you want a value of 1.3 - 3.0. Below 1.3 suggests the projectile will not stabilize with the given twist/velocity. Above 3.0 suggests your accuracy might improve by using a slower twist. The 175gr MK has a stability value of 1.173 @ 2600 FPS in a 1x12" barrel, which is below marginal. It's 1.335 in a 1x11.25" barrel, which is acceptable. A 1x10" barrel yields 1.689 which is much better.

    Similarly, the 168gr (which has a length of only 1.21" as opposed to the 175gr's 1.4") stabilizes just fine in a 1x12" barrel with a value of 1.717. But it will not stabilize it at subsonic velocities (need at least 11.25" for that).

    I was a bit skeptical about this formula before but after using it for a while it seems to be fairly sound. It's much better than guessing.

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    Re: Twist for 308 WIN???

    Originally Posted By: .257
    It seems most guys run the 175 grain bullets in the 308 Win.

    Why are most people running a 1/10- 1/11 twist???


    I actually suspect there are more 1:12s than 1:10 from manufactures....at least numerically

    Quote:
    If a 1/13 will work


    1:13 will work if you drive them very hard or live/shoot at altitude (above 1000 feet). However, if you don't drive them hard (>2500 fps) they may not stabilize (may not).

    Quote:
    A 1/12 or 1/11.25 seems like a better choice. I have always heard it is better to over stabilize than under stabilize. It seems though that shooters should run a optimium twist rate. If this does not hold true we would all shoot with the fastest twist rate avaliable.

    I am about to have a 308 WIN built. I do not plan on going heavier than a 175. The 175 is my first choice and the 155 my second.


    The guys who shoot long distances for score (like PALMA, F-class) want just enough stability so that the bullet will nose down during the downward decent to the target. This can tighten up groups from 800 yards and out, at some expense to the up close groups inside 300 yards. This is a specialized shooting arena and every little bit helps.

    For tactical purposes this may not be the ideal trade off, but if you are shooting only one load in your gun and develope a complete set of dope for both gun and load, then it remains viable. However, if you like to experiment with loads and bullets, the faster twist rate leaves so little on the table accuracy wise, and desensitizes the gun to so many effects, it is typically the better trade-off for general purpose shooting.

    1:13 a specialized twist rate for long distance shooting
    1:12 a general purpose twist for bullets up to 175
    1:11 a general purpose twist for bullets up to 200
    1:10 a specialized twist rate for bullets above 130 and below 220

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    Re: Twist for 308 WIN???

    Originally Posted By: AshesToAshes
    See Don Miller's stability formula here:

    http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi

    Ideally you want a value of 1.3 - 3.0. Below 1.3 suggests the projectile will not stabilize with the given twist/velocity. Above 3.0 suggests your accuracy might improve by using a slower twist. The 175gr MK has a stability value of 1.173 @ 2600 FPS in a 1x12" barrel, which is below marginal. It's 1.335 in a 1x11.25" barrel, which is acceptable. A 1x10" barrel yields 1.689 which is much better.

    Similarly, the 168gr (which has a length of only 1.21" as opposed to the 175gr's 1.4") stabilizes just fine in a 1x12" barrel with a value of 1.717. But it will not stabilize it at subsonic velocities (need at least 11.25" for that).

    I was a bit skeptical about this formula before but after using it for a while it seems to be fairly sound. It's much better than guessing.


    This will cause people to think that a higher value is better. That is not the case. I am of the opinion that one should use the slowest twist that will statically stabilize the bullet at the muzzle. One needs to understand why spin is required in the first place and then how this changes during the flight of the bullet.

    The spin rate is a function of the barrel twist rate and muzzle velocity. The spin is required to counteract the overturning effect air resistance has on the bullet, because of the elongated shape of the bullet. The greater the air resistance the greater the overturning forces and thus the faster the required spin rate to act as gyroscopic device. Also, the longer for caliber the bullet is, the greater it propensity for overturning, so it needs a higher spin rate to stabilize. Because spin rate is a function of twist rate and muzzle velocity, you can sometimes make up the required spin rate by upping the velocity, but that also cause the required spin rate to increase, albeit more slowly.

    The downside of a high spin rate is that it can induce secondary instability into the bullet if said bullet is not perfect. This will manifest itself with precession that in more extreme case may even cause the bullet to deviate from its course and open up the group. This is why benchresters use "perfect" bullets at the lowest spin rate needed to stabilize.

    Once the bullet leaves the muzzle, if it is statically stable at the muzzle, it will be stable all the way, until it reaches either the target or transonic velocity. During the transonic passage, the bullet may become unstable, like the celebrated case of the 168gr SMK, but not always. Most bullets will remain stable. However, before the transonic range or the target, is attained, the bullet actually gains stability because the forward velocity decreases and the spin rate does not. As it approaches the transonic range, the pressure does start to build up again. This is why the bullet should be supersonic when it gets to the target and the faster the better.

    All this to say that I would not simply get a 1:10 twist if I would never use bullets that required the spin rate generated by that twist. I would get a twist rate that will properly (but not overly) stabilize the heaviest (longest) bullet I am going to use.

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    Re: Twist for 308 WIN???

    Originally Posted By: MitchAlsup
    Originally Posted By: .257
    It seems most guys run the 175 grain bullets in the 308 Win.

    Why are most people running a 1/10- 1/11 twist???


    I actually suspect there are more 1:12s than 1:10 from manufactures....at least numerically

    Quote:
    If a 1/13 will work


    1:13 will work if you drive them very hard or live/shoot at altitude (above 1000 feet). However, if you don't drive them hard (>2500 fps) they may not stabilize (may not).

    Quote:
    A 1/12 or 1/11.25 seems like a better choice. I have always heard it is better to over stabilize than under stabilize. It seems though that shooters should run a optimium twist rate. If this does not hold true we would all shoot with the fastest twist rate avaliable.

    I am about to have a 308 WIN built. I do not plan on going heavier than a 175. The 175 is my first choice and the 155 my second.


    The guys who shoot long distances for score (like PALMA, F-class) want just enough stability so that the bullet will nose down during the downward decent to the target. This can tighten up groups from 800 yards and out, at some expense to the up close groups inside 300 yards. This is a specialized shooting arena and every little bit helps.

    For tactical purposes this may not be the ideal trade off, but if you are shooting only one load in your gun and develope a complete set of dope for both gun and load, then it remains viable. However, if you like to experiment with loads and bullets, the faster twist rate leaves so little on the table accuracy wise, and desensitizes the gun to so many effects, it is typically the better trade-off for general purpose shooting.

    1:13 a specialized twist rate for long distance shooting
    1:12 a general purpose twist for bullets up to 175
    1:11 a general purpose twist for bullets up to 200
    1:10 a specialized twist rate for bullets above 130 and below 220


    Nice write up. Thanks

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    Re: Twist for 308 WIN???

    Lilja has a good chart that gives suggestions on twist based on caliber and bullet weight. http://www.riflebarrels.com/products...wist_rates.htm
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    Re: Twist for 308 WIN???

    I have know Idea why but everyone that I talk to says 11.25 is the best twist for 175's

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    Re: Twist for 308 WIN???

    One thing no one has mentioned yet which was a big consideration for me, is the ability to stabilize subsonic rounds. I like to plink with subs and with a suppressor, it's just too much fun.

    The 1:10 twist gives you a bit better margin stabilizing bullets through a suppressor. With a 1:10, 180gr round nose soft points are stable at subsonic speeds with a good degree of safety.

    This twist has served me perfectly fine as my go-to load is the 175gr Sierra at 2650fps. I have fired the 155gr Scenar with good results also, but I just prefer the 175's.

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