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Old June 20, 2009, 09:49 PM   #1
Blue
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308 Win - Best Barrel Length and Twist

For the Sierra BTHP 168 gr --- what is the most favorable (for accuracy 500 yds) "heavy" barrel length and twist -- Model 70 Win. Same question for the Rem 700 + "heavy barrel".

Savage has some now out to 30" - is there an advantage to this.

Thanks,
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Old June 21, 2009, 09:24 AM   #2
sadsack
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12" twist has worked best for me with 168gr. Lenth depends on personal preference and what style of shooting you intend to do, 26" or 27" at the most.
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Old June 21, 2009, 12:50 PM   #3
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1:12 20-22 in. barrel for a hunting rig works best for me. Shooting 150-165 gr. bullets.
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Old June 21, 2009, 02:25 PM   #4
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I would get an 11" twist, especially if you want to use the newer 175 grain SMK. The M24 Sniper system is designed for the 175 and uses an 11.2". Also, the M14 was started out with a 12" twist, but the match armorers changed them to 11" when building guns to shoot the old 173 FMJ match boattail and for the M852 ammo that used the 168 grain Sierra MK.

The reason for choosing the 175 grain over the older 168 grain design is it is stable through the transonic region, so it can be shot out to 1200 yards and beyond. The 168, which was originally designed for 300 m International rifle match shooting, becomes unstable in the transonic region and will start to tumble beyond 700 yards when fired from a .308 or a .30-06 with conventional barrel lengths.

The .30" .308 barrels are for Palma match rifles where the rules don't allow the bullet weight to exceed 155 grains and so they need every last fraction of additional speed they can get so the bullet will get to 1000 yards withous dropping subsonic. They have typically had 13" twists on those barrels because of the length of the old 155 grain SMK and lighter bullets used in the past, but the Lapua and the newer Sierra designs for that weight are longer and have higher BC's. I don't know if that has caused them to move to 12" twist rates or not? I don't keep up with that particular discipline.
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Old June 26, 2009, 06:26 PM   #5
Blue
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175 gr bthp

Would 12" twist still be ok for .308 175 gr, also. It is said the 175 gr remains more stable through the trans-sonic phase past 700 yds. I haven't tried the 175 gr yet and am wondering how it would work up for point blank (100yds) out to 600. Have had good luck with the 168 gr. out to 600.
Thanks.
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Old June 26, 2009, 10:12 PM   #6
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barrel twist

I believe Greenhill discovered a formula for the amount of twist required for bullet stabilization. Kinda long, so I made it an attachment. Never used it yet, so I can't speak from personal experience. Perhaps others here have?
Attached Files
File Type: txt GreenhillTwistFormula.txt (5.2 KB, 553 views)
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Old July 3, 2009, 10:57 PM   #7
G24=40cal
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I have a Savage 10 24" Bbl 1-10" Twist, anything from 110gr V-Max, 150gr
BT, 168 SMK, 175gr SMK and 190gr SMK all shoot very good in my rifle.
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Old July 4, 2009, 12:49 PM   #8
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Thanks asskickinpeanuts, I was looking for that formula, you just saved me a few steps of looking.
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Old July 4, 2009, 04:05 PM   #9
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The Greenhill Formula

Is a work of art, for its time. The main limitation to Greenhill is his work was all done with black powder weapons - cannons in fact. This limits the velocity range to around 1800 to 2100 fps. The second limitation is he only had pretty much cylindrical with rounded or pointy end projectiles.

The limitation doesn't make it wrong, just limited. The Greenhill formula is still a pretty good rough guide to twist rate need. However, I've never been able to discover if the formula gives the minimum twist needed for projectile stability or the optimum twist for projectile stability. My guess, based on using the formula and my own experience, is it is the minimum rate. (Participants are free to disagree.)

Concerning the velocity factor limitation. Greenhill only used two constants in regard to velocity; one for less than 1800 fps and one for over 1800 fps. I rather think that constant - actually a modifier - changes with greater velocity. As velocity increases, the actual spin of the projectile increases in terms of RPM, generating greater stability. Simultaneously, atmospheric resistance increases and therefore a tendency to yaw in flight, generating lesser stability. The lessened stability outweighs the greater stability, so the twist rate must be altered. However, not being independently wealthy or having a government grant, I cannot currently explore the mathematics involved.

Also missing from the equation is the concept of ballistic coefficient. In Greenhill's day, all projectiles were either round balls, or a cylindrical body with a more or less pointy front. Spitzer, long radius ogive and boat-tail bullets change the equation. Again, I cannot currently explore this phenomenon.

Still, Greenhill is a good place to start. It is in fact, the best starting place we have.

I do know this much:

The M14 has a 20 inch barrel and delivers a pretty good velocity for general use. I would consider 20 inches to be the minimum for a general or sporting use rifle.

Most of the .30 caliber rifles run about a 1 in 10 twist. For most purposes, that's a bit over spun.

The longer (heavier, usually) the bullet, the more spin is required.

A lot of overspin is better than just a little underspin. If one errors in choosing a twist rate, err on the faster twist side.

There! The benefits of my vast knowledge and experience. You're on your own, bunky.
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Old July 6, 2009, 05:21 PM   #10
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1-11.5 and a 26 inch barrel make a great combination. This is on a 700 action.
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Old July 9, 2009, 06:10 AM   #11
Bart B.
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For many years, 7.62 NATO converted accurized Garands with 1:12 twist 24-inch barrels shot 168 through 190 grain Sierra HPMK bullets with match winning accuracy up to 1000 yards. Accuracy in good rifles with good ammo was 4 inches or better at 600 yards.

In bolt action match rifles, 26 inch 1:11 twist barrels chambered for the .308 Win. did the same thing. Some of these rifles have shot groups smaller than current bench rest records just testing for accuracy.

Make sure the barrel's groove diameter is 4 to 8 ten thousandths smaller than bullet diameter. If you want to use 168's, you'll need to drive them out at least 2700 fps else they'll go sub sonic at about 900 yards at elevations below 2000 feet. Shooting 168's in 22 inch barreled M14/M1A rifles at 1000 yards too often didn't shoot 'em fast enough. Slightly oversize groove diameters plus shorter barrel length led to the demise of 168's in Lake City M852 match ammo with them. At the 6600 foot elevation of the NRA's range at Raton, NM, M852 ammo from these short barreled rifles stayed supersonic through 1000 yards. Not so at the 600 foot elevation of Camp Perry where the Nationals are fired.
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Old July 9, 2009, 09:43 AM   #12
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A 1:10 twist shoots everything well, from 125's to 190's.

I have a number of 26" 1:10 308 barreled rifles. Basically because that was what everyone used. Now I find a 24" tube to be handier.

A 30" barreled rifle would be a monster to carry and store. Might as well try carrying a 10 foot pole and see how that gets in the way.

Where do people get carrying cases for those things?
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Old July 9, 2009, 12:02 PM   #13
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I would say between 20 and 24" with an 1:11.25
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Old July 9, 2009, 02:02 PM   #14
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I read an article in Benchrest Primer that said 1/14 twist was perfect for 168gr SMK in Hunter Bench rest.
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Old July 9, 2009, 06:04 PM   #15
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If you want an update to the Greenhill formula that considers temperature and bullet weight and velocity and barometric pressure in addition to the Greenhill factors, Don Miller published one in Precision shooting long ago. It seems to work pretty well outside the transonic range. I made it into an Excel file that calculates gyroscopic stability factor, s, using Miller's formula and it recommends a twist for an entered stability factor, and also lets you put in a twist to see what stability factor it creates? You can download it free from my file repository, here. It will also work with the free Open Office Suite's spreadsheet program, Calc. I also have an altitude barometric pressure estimator below the stability estimator. I put a second worksheet in the file that calculates actual stability factor from win drift diagonal in sets of five shots. Shoot a number of such groups and average the results to see where your barrel actually puts the bullet?

I should amend my earlier 11" recommendation to point out that the military tries to pick twists that will work all the way down at lowest possible altitudes and in the coldest temperature conditions, so they tend to overstabilized by most civilian standards. If you know you will not be shooting below sea level and will not be shooting below the 59 degree standard metro conditions, then a 13" twist will stabilize the 168 almost optimally by Harold Vaughn (s=1.4) and Don Miller's (s=1.5) criteria. It is worth pointing out that "overstabilizing" is only a fault if your bullets are imperfect so that the spin makes them wobble eccentrically, or if you are driving them so fast they could fly apart, or strip their cores from their jackets due to excessive rotational acceleration. Otherwise, there is no bad effect to extra twist. The modern match bullets are made very well, indeed, and that is why even guys with the old Garand and Springfield 10" twist barrels don't have problems shooting the 168's down to half moa or so with a tuned load and rifle.

As to the 168 gr. SMK's stability, I first saw them tumble while shooting at a 748 yard popper at Gunsite back in 2000. These were PMC match ammo fired from a 22" tac rifle barrel, and were probably going about 2550 fps or so. At the time I didn't realize the misses were due to tumbling. We were shooting across a valley with a stiff wind and figured there must be bad gusting on the far side that we couldn't see through the steady mirage nearer our end. Wrong.

The same year I attended the Long Range Firing School at Camp Perry. The first range session was at 800 yards, and as firing commenced it was accompanied by a lot of howling from the line as reports of keyholes came back from the pits. None of us could stay on paper shooting the 168's.

Sierra ballistics technician Kevin Thomas was attending the school, too, that year, and he stated the 168 had originally been designed just for 300m International match shooting. That it also worked to 600 yards for service rifle matches had turned out to be a bonus. He suggested the newer 175 didn't have that stability problem in the transonic range, and we should switch. So, at lunch time Commercial Row was raided for every loaded round featuring the 175's we could find. I got some HSM from O.K. Weber, and it worked just fine to 1000 yards.
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Last edited by Unclenick; July 9, 2009 at 08:12 PM.
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Old June 1, 2014, 09:48 PM   #16
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Don Millers updated Greenhill formula.

I would love to see Don Millers updated Greenhill formula.
If you could please update the link, I would appreciate it.
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Old June 2, 2014, 02:26 AM   #17
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Please define "accurate".
Some shooters will say "my rifle is very accurate" but don't define that with a number.
Could you say what your rifle will do at 100 yards, 500 yards and so on.
Could you mention loads? No need to be specific about the powder weight, just what powder.
I have two rifles in .308, one a Remington R25 (AR)
another a "tactical" Remington 700 with short, 20" H-bar that can shoot
1/2 inch IF I am having a good day at the bench.
Federal 168 grain match ammo will only group 1.5 inch at 100 for
5 shots, Hornandy 150 SST, shoot a satisfying 1/2 inch on a good day.
Even with the short h-bar the 700 chrono's a tad over 2800 f.p.s. with the
150 SST bullets & 2000 MR powder.
(that with loads from the Hornady manual)
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Old June 2, 2014, 09:24 AM   #18
Bart B.
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Several good calculators are in this web site:

http://www.jbmballistics.com/ballist...culators.shtml

The stability one's probably the best one to use for barrel twist figuring.

Best accuracy has always happened with a bullet shot at some muzzle velocity is spun just fast enough to keep its long axis parallel to the trajectory all the way to the target; point on, in other words. Any faster twist tends to make the bullets that are a tiny bit more out of balance (maybe 1 out of 100 is perfect) than the others nutate or cone too much. That increases drag as well as lets them jump more off the muzzle axis upon exit due to centrifugal forces. Short range benchresters adjust powder charge weights 1/10 grain or so to change muzzle velocity for atmospheric conditions. Cooler weather means thicker air and the bullets need to leave a tiny bit faster to be perfectly stabilized all the way to 300 yards.
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