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Thread: Twist ratio vs bullet weight???

  1. #1
    Member Magnum Man's Avatar
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    Default Twist ratio vs bullet weight???

    Can anyone enlighten me on this? How does the twist ratio effect the bullet weight you can use? This is regarding The DPMS AR I will be getting soon. It has a 1 in 9 twist.
    Thanks

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    Others will have more info on this than I, but what I understand is, that a barrel with a higher twist ratio (more twist) gives the bullet more spin. The more spin the bullet has, the more stable it will be in flight. It's the old football example. A football thrown with a tight spiral flies alot farther and more accurately than a wobbler. The same is true with bullets, so any barrel that can put more spin on a bullet is a good thing. A heavy bullet will require more rifling to give it sufficient spin because the heavier an object is, the more it requires to move it from rest. The length of the bullet also makes a difference, because a longer bullet has more contact with the rifling, and therefore more spin.

    Like I said, that's my two bits. I'm sure Murphy and others will have better info.

    Take care,

    Terry

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    The 1 in 9 twist is good for up to 70gr bullets. That is the upper range though, and is optimized for the 55-69 grain range. Most varmint 223's that are shooting the really light bullets (40 gr) have a 1 in 12 twist.

    Hope this helps.

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    MM,maybe this will help:
    http://stevespages.com/page8e.htm

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    Member Magnum Man's Avatar
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    Lightbulb Thanks

    One more variable then, would be the faster the bullet is moving also the more spin it would have compared to the same bullet at less velocity?

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    It isn't really true that a bullet of 70 grains neds this twist or a bullet of 55 grains needs this twist.

    The stability requirement for a bullet is based on its length in calibers. If a bullet is 2 1/2 times longer than its diameter it requires a certain rotational speed, surface feet per miunute, to be stabilized. We can get the is rotational speed by using a certain twist rate in the rifling and then launching it at a certain velocity. If velocity is faster, we can use less twist of rifling. The 22-250 is faster than the 223 and uses a 1 in 14 twist to stabilize lead core (a certain length ) 55 grain bullets. The 223 needs a twist rate of 1 in 12 to stabilize the same bullet. Generally the 1 in 9 twist will stabilize lead core 70 grain bullets...if the muzzle velocity is kept high enough. When using shorter barrels, 14 to 16 inches, this 70 grain lead core bullet won't be stabil much beyond about 200 yards.

    I've shot the 69 grain Sierra hpbt, a particularly long bullet, with a 1 in 8 twist to good groups at 300 yards from a 16" 223 chambered AR. The same results from a 20" barrel with 1 in 9" twist. Both may be good beyond that range but there will be a range limit when velocity drops.

    Also bullet velocity drops proportionally with BC but rotational velocity has a much slower decay. What that means is that twist rate will work to lengthen the reach better than velocity. The design of the 1 in 7" barrels was for the M855 round to be stabil at 500 meters from the 16" barrel. This bullet is very close to the length of the Sierra match hpbt bullet.

    If I have a 16" barrel and want to shoot 69 grain hpbt (longer than regular spitzer) at 300 yards and beyond, one in 8 twist is enough for the 223, 1 in 7 of course will work and a 1 in 9 might be ok. With more barrel, I can use less twist and vice versa.
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    One rough and loose way to calculate the twist needed is to use
    Caliber in inches, times
    31 heavy long bullets
    35 heavies
    41 medium
    55 med to light
    63 short and light

    So .223
    x 31= 6.9 twist
    x 41=9.1 twist
    x 55= 12.2 twist
    x 63= 14.09 twist

    For 30 caliber

    .308
    x 31=9.5 twist
    x 35= 10.7 twist
    x 41= 12.6 twist

    .458 caliber
    x 31 = 14 inch twist
    Floatplane,Tailwheel and Firearms Instructor- Dragonfly Aero
    Experimental Hand-Loader, NRA Life Member
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    Google-up "Greenhill Formula" and you'll get your answer...which I learned from Professor Murphy a few years ago.

    Doc

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    The Greenhill formula is well known as are Murphys' writings on various forums regarding this subject so I won't attempt to add to either. I do have some experience with the AR platform and the SDM rifles as I was involved in that program at Fort Steward a few years back. We settled on the Sierra made 77 grain bullet, essentially a hollow point boat tail match bullet. The original Black Hills loading of the Mk262 smooth bullet for the Navy contract, for the M12 system, was used there when we started the DM program. The M855 62 grain green tip proved less than satisfactory at the long line at Stewart. This was later modified with the addition of a cannelure and designated the Mk262 mod 1, this is the load in use today.

    These highly modified rifles used 20" barrels with 1 in 7" twists and later with 1 in 8" twist, were capable of .5 MOA groups at 900 yards. I believe the rifles were fielded with both configurations and 7 or 8 inch twist were both good enough to score possible scores on the 900 yard range. This load is loaded to higher pressure than the M855 and requires some changes in the rifles but it is a very capable rifle now with these changes.

    This bullet is more frangible than the M855 and much more accurate but penetration on hard targets is also much less. This program was meant to provide squad designated marksmen with a rifle and load to bridge that gap from 300 yards to 600, beyond the range of aimed fire form the 7.62x39, it has been successful.

    As has been mentioned the 1 in 7" twist is needed for the longest bullet, barrels and ranges. For 300 yards with the 69 grain match bullet a 1 in 9" is all that is needed and I've shot that combo out to 600 yards with a 20 inch barrel on my match rifles. It doesn't hit with authority but it does make nice round holes in the ten ring.

    I would say for a good general duty 5.56 rifle a twist of 1 in 9" is more than adequate but if you plan to compete at the long line with the long 77 grain bullets, an 8" or even 7" twist would be needed.
    Mike
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    "The rifle brought man out of the mud". Cooper

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