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Tech. question: 30 cal twist rate

Discussion in 'Shooting Related Threads' started by Shooting Coach, Dec 26, 2010.

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  1. Shooting Coach

    Shooting Coach Well-Known Member

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    Past 800 meters with the 308, you are out of gas and very susceptible to wind.

    For the purpose of discussion, the 1-10" twist would be more suitable for the 175-178 gr bullet past 550 meters. The 175-178 gr bullets of suitable design overtake the 168 at ranges exceeding 600 yards.

    My last 30/06 Service Match Garand had a 1-11" twist.
     
  2. chipking

    chipking TS Member

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    1 in 12 is marginal for 168gr @ 800 yards unless you are using one of the 300 magnums and start the bullet faster. I used my same 308Win M-70 with a Hart 1-11 twist for High Power and 1000 yard matches and really had to load to the top limits with 168gr to get any stability out there. I actually had much better groups with 155 Lapua at 1000yds in that slow twist. I'm pretty sure that the Marines I was shooting against in the 1000 yard matches were using 1-12 in a 30-338 Mag. and a 180gr bullet.

    --- Chip King ---
     
  3. jhmorrisn

    jhmorrisn TS Member

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    Tinman

    Tell you what you should try.

    Load up several loads of what ever you got.

    Go to your local range and from the 200-yard line on a NRA SR target, see how many shots you can put in the 10 or X ring. At 200 yards the 10-ring is 7 inches and the 10 is 3.

    When you done that go to the 600-yard line. Most high power ranges but not all have a 600 but none that I know of have a 800 FL.

    At the 600 on the MR-1 target with it’s 12” 10-ring and 6” X-ring, see how you do on that one.

    Now when you find 800 yard is requirement, it’s only 3 or 4 click away from 600,

    Jim

    PS BTW, do you shoot a pellet rifle.
     
  4. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    The military uses 175gr. bullets in the M24 and M40 using a 1-11.7 twist barrel and they work fine even beyond 1,000 yards. The 168-175 SMK have to leave the barrel at 2,700+fps to still be supersonic at 1,000 yards, you also need a minimum of 24" barrel to get those velocities. correction the twist on the M24 and M40 is 1-11.25
     
  5. loop02

    loop02 Member

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    As long as all the twist experts are on here, what do you think of 1 in 9 twist for a 6.5/.284 shenhane. I got a couple of barrels, but now I hear 1 in 8 is the hot setup. It will shoot around 3000 ft./sec/, with 140 grain bullets. 28" barrels
     
  6. hmb

    hmb Well-Known Member

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    The heavier the bullet you want to use the faster the twist rate has to be to stabilize the bullet. It also helps to keep the bullets velocity above the speed of sound until it reaches the target. Otherwise accuracy will suffer when the bullet passes through the sound barrier and encounters turbulence. HMB
     
  7. Texshooter

    Texshooter Member

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    168 Gr Maybe! 175 less likely, if it's already built... go try it. AJ
     
  8. halfmile

    halfmile Well-Known Member

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    Longer (therefore heavier) bullets do indeed require more spin. Every bullet has an RPM range it needs to operate in in order to achieve best accuracy.

    The 155 Palma likes to turn at 145,000 to 170,000 RPM. The 180 Match King likes 160,000 to 190,000 RPM.

    You can calculate RPM with this formula:

    Muzzle velocity times 720

    ------------------------- = RPM

    Twist rate in inches





    Most good bullets like the upper end of their RPM range.

    Unfortunately, bullet manufacturers do not give info on this subject. Sierra does give different BC figures for different speeds on some bullets.

    Credit to Bart Bobbit for the above info (Usenet: alt.rec:guns)

    HM
     
  9. hmb

    hmb Well-Known Member

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    One of the most accurate barrels I ever had was a Hart 1 in 14 inch twist 308 caliber barrel. Used it for 100 and 200 yard NMC competitions with 168 grain Sierra Match King bullets. HMB
     
  10. W.R.Buchanan

    W.R.Buchanan Member

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    Most 30-06's run 1 in 10 twist barrels. Most Garands are 1 in 10 and prefer 150-155gr bullets. I have a Garand with a NM barrel which is 1 in 9 and will shoot 168gr bullets well. But all of this goes out the window as soon as newer solid metal bullets are substituted.

    It not so much the weight of the bullet, it is the length that governs the twist requirements. The new all copper bullets out there are longer than their same weight Conventional Bullets, as a result they are requiring faster twist rates than previously considered normal.

    AR's now have 1 in 7 twist barrels available made to shoot 80-90gr+ .22cal bullets for long range match use, however if you try to run small bullets like 45-50 gr varmint bullets in these fast twist barrels they blow up from centrifugal force as soon as they clear the muzzle. Poof.

    On the other end of the spectrum,,,

    My Marlin .44 Magnum Rifle and Carbine both have 1 in 38 twist barrels and will not stabilize any bullets longer than about 265 gr. The loose bores at .431 doesn't help either. They are both made to shoot short fat bullets, and they don't even do that particularly well. In fact both of these guns have abismal accuracy which is why they are both going to be rebarreled with 1 in 20" twist barrels. Then they will both shoot up to 330 gr bullets accurately. Right now 3" at 50 yards is a good group. 1.5-2" at 100 yds is more what I'm looking for. With the 1 in 20 barrels I should be right in there. I want to be able to run 250 gr bullets at 2000 fps and 300 gr bullets at 1800. This is 45-70 power in a 36" long carbine,, Which will also have a decelerator pad and maybe even one of those Mercury doodads in the stock for good measure. Good companion for a .44 Bisley Revolver and able to shoot the same ammo.

    This is a fascinating subject and I don't think you can have too much info, as it is the very essence of the rifle. Why and how they determine the twist rate for a given bullet is not clear. I wonder if it is done by trial and error or if they can predict based on length versus diameter?

    Randy
     
  11. shot410ga

    shot410ga Well-Known Member

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    Shooting Coach: I suggest you attend an "F" Class TR event. You will see guys shooting the X ring at 1000 yards without any problems with the .308, in all sorts of wind. Sometimes, I wonder about your name....
     
  12. TOOLMAKER 251

    TOOLMAKER 251 Active Member

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    Randy, I have several ar rifles with twist rates from 1-7 to 1-9 My Colt H Bar is a 1-7 twist and it will shoot 52gr. SMK's at 3,400 fps. just fine, no bullets blowing up.
     
  13. g7777777

    g7777777 TS Supporters TS Supporters

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    There are manuals and thousands of opinions on this subject- read all of those-and you wont have the answer

    The answer is

    "it depends"

    regards from Iowa

    Gene
     
  14. halfmile

    halfmile Well-Known Member

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    It started with the Greenhill formulas about 100 years ago. This was in the time when most rifle bullets were in the 1300 to 1900 FPS range.

    Leaps and bounds in this science came about because of missile and aeronautic technology improvements. The fact that match competitors were all mad scientists using incantations and potions in their basements also helped.

    These days Google is indeed your friend. A few minuts online can take the place of much library and magazine searching.

    HM
     
  15. Old Cowboy

    Old Cowboy Active Member

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    Can a bullet be overstabilized?
     
  16. hmb

    hmb Well-Known Member

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    There is a combination of spin rate and velocity that affects accuracy. There is a sweet spot where the bullet is very stable and accurate. Remington used to send a test target with their match 308 rifles, 37 grains of 3031 and a 168 grain match bullet, that was their sweet spot load. HMB
     
  17. Old Cowboy

    Old Cowboy Active Member

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    So WHY..........is accuracy adversely effected if the spin rate vs velocity (rpm?) is too high?
     
  18. Remstar311

    Remstar311 Member

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    1 in 7 twist AR's have been around for a long time. They also don't blow up any 45-50 grain bullets. It likes the heavier ones better.
     
  19. V10

    V10 Well-Known Member

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    John,

    Some people believe that a bullet that is "over stabilized" will fly at the same orientation throughout its flight path and thus "key hole" on the target, i.e., maintain a nose up attitude.

    The above link looks like a good discussion of the topic.
     
  20. W.R.Buchanan

    W.R.Buchanan Member

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    toolmaker: In the 1/7 barrel at 3400 FPS those bullets are spinning at 349,714.27 RPMS! That's pretty fast, and whereas your bullets might not be blowing up, many light jacketed varmint bullets won't live thru that much Centrifugal force. 350 grand is whingin' I don't know of anything else that spins that fast, do you?

    1/9 is only 272,000 Rpm's at 3400fps. I could see alot of bullets surviving that.

    By Comparison those F Class bullets are only turning 168,000 rpms at 2800fps from a 1/12 barrel.

    These numbers are mind boggling but they are accurate. People don't realize the bullet will only be flying for 1 or 2 seconds, and might only make 4-5000 turns in it's flight.

    Formula is; twist rate divided into 12 X FPS X 60 = Bullet RPM's

    Example for 1 in 7 twist: 12/7 = 1.714 rotations per foot. Times 3400 FPS = 5828.57 rotations per second, 5828.57 RPS x 60 = 349,714.27 RPM.

    Randy
     
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