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O/T rifling twist & accuracy

Discussion in 'Uncategorized Threads' started by Old Cowboy, Dec 28, 2007.

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  1. Old Cowboy

    Old Cowboy Active Member

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    Can a bullet be spun too fast? I'm aware that using a heavier than normal for caliber bullet in a barrel of standard twist for that cartridge can cause a decrease in accuracy due to insufficient bullet stabilization.

    Can using too LIGHT a bullet also cause accuracy problems? If so---why??

    John C. Saubak
     
  2. Shooting Coach

    Shooting Coach Well-Known Member

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    Dear John

    The answer is yes to both questions. The best example might be a 40 grain bullet fired in the 5.56 AR barrel with a 1-7" twist. At range, the bullet will precess like a gyroscope. This may be at several hundred yards.

    If the 80 gr Sierra MK is fired in a 223 with a 1-14" twist, it will have an uncontrolled keyhole quickly.

    A 110 gr bullet fired from a 30/06 with a 1-10" twist would not be as accurate as the same bullet fired from a gun with a 1-14" twist. However, a 30/06 with a 1-14" twist would be very limited in its use.

    Standard weights in standard calibers is the key. If you want a wildcat or desire to shoot a very light or heavy bullet, do your research, and talk to barrel makers. If a twist is not available, there is likely a good reason.

    Typically, extended range performance is enhanced by use of aerodynamically efficient heavy bullets.

    What do you have in mind?
     
  3. Mike Michalski

    Mike Michalski Member

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    John, You can spin some bullets so fast they disintegrate just out of the barrel.
     
  4. hmb

    hmb Well-Known Member

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    A 30-06 with a 14" twist is a very accurate combination, it will stablize everything up to and including 168 grain bullets. HMB
     
  5. grunt

    grunt TS Supporters TS Supporters

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    Bullet weight determines the twist needed.
     
  6. halfmile

    halfmile Well-Known Member

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    Grunt, you are right, buuuuut..........

    What stabilizes the bullet is RPM's. This is a function of velocity and twist together.

    I'm thinking of a 6mm AI that works a 70 grain bullet well, even though it has too slow a twist (14). The 3700 plus velocity produces the RPM that a faster twist would at slower velocities.

    If you go to 6mmBR.com there is a forum there that has lots of very good Rifle guys.

    I have a couple essays on the subject for you if you email me.

    HM
     
  7. Brian in Oregon

    Brian in Oregon Well-Known Member

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    "Bullet weight determines the twist needed."

    Wrong. Bullet DIAMETER and LENGTH determines the twist needed.

    Twist = 150 X D²/L

    Where: D = bullet diameter in inches L= bullet length in inches

    Click the link for more info.
     
  8. m70win

    m70win Member

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    That's true Brian a short flat base round nose won't key hole when a longer hollow point boattail will.

    Tim
     
  9. Brian in Oregon

    Brian in Oregon Well-Known Member

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    BTW, the Greenhill Formula (see the formula in the previous post) is not 100% exact. It will work, but the result can be 'tweaked' for specific conditions.

    For example, it's common for .45-70 rifles that shoot 500 grainers to add 2" to 4" to the calculated twist rate because of the slow velocity. Generally a calculated 20" to 22" twist will be reduced to 18".

    And often the .22-250 has its calculated twist rate slowed by 2" because of overstabilization due to increased velocity. For example, though the 55 rain bullets for a .223 are ideal in a 12" twist, many 22-250's have a 14" twist.
     
  10. grunt

    grunt TS Supporters TS Supporters

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    Brian, Your right I over looked the length.
     
  11. Old Cowboy

    Old Cowboy Active Member

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    I'm working with a new .375 Ruger, "Hawkeye African". She'll shoot just slightly over 1 moa (5 shot groups) with Hornady factory 270 gr spire-point loads. Trying to develop a 225 gr SP handload with IMR 4064, best I can come up with so far is twice the size groups (over 2"). I'm thinking~~~~~~might be too tight a twist? Might be something else??

    John C. Saubak
     
  12. Brian in Oregon

    Brian in Oregon Well-Known Member

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    John, keep in mind that group size can be affected by barrel harmonics. If you can't get a good group with a certain powder with incremental steps in amount, try a different powder.
     
  13. Old Cowboy

    Old Cowboy Active Member

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    Thanks Brian,

    I might try IMR4320, or maybe IMR4350

    This cartridge is so new tho' that it's hard to find loading data for it?

    John
     
  14. Shooting Coach

    Shooting Coach Well-Known Member

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    The 375 Ruger might like Alliant's RL-15 with the lighter bullet. RL-22 might be worth a try. These powders do very well in inclement weather. A federal 215 appears to be in order. This gun is not a long range match gun, but Thor's Hammer in a standard length action!

    The poster commenting on the 30/06 being very accurate with the 1-14" twist is right, but one loses the ability to shoot 180 grainers, the pill that made the 30/06 famous. I personally shoot the 165 Hornady BTSP for game in my area. This is one of the few times I am happy with a compromise between the 150 and the 180. For target, I have several thousand 190 SMK I bought from a Military shooter who was deployed to the sand box. The fast 10" twist of the '06 is a carry over from the 220 grain bullet of the 30/40. However, the big bullets shoot great in the '06.

    When I shot the Service Rifle Match Course I had stainless Krieger barrels on my Garand with a 1-12" twist. It was fed exclusively the 168 SMK and Accurate Arms 2520. This is one of the finest powders I have ever used in the 30/06 and 308 with the 168, and the 5.56 with the 69 gr SMK.
     
  15. hmb

    hmb Well-Known Member

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    When you would buy a Remington match rifle in the old days it would come with a test target fired at 100 yards. Two or three 5 shot test groups were shot with 37 grains of 3031 and a 168 grain match bullet. I shot alot of good scores using that load out to 200 yards. HMB
     
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