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 1 IN 12 twist, 30 cal
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longshot
Junior Member

USA
61 Posts

Posted - Jan 22 2004 :  18:29:07  Show Profile Send longshot a Private Message  Reply with Quote
Just wondering what bullets stablize best in a 1 in 12 twist .308 caliber rifle? I shot some 180 grain bullets at the range this weekend and found they stabilized at lower velocites; at around 2,300 FPS. At higher velocities my groups opened up quite a bit.

Thanks

Robert

ricciardelli
Advanced Member



USA
1581 Posts

Posted - Jan 22 2004 :  21:55:49  Show Profile  Visit ricciardelli's Homepage  Reply with Quote
http://stevespages.com/page8e.htm



http://stevespages.com/page8c.htm

Edited by - ricciardelli on Jan 22 2004 21:56:25
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Paul B
Advanced Member

3300 Posts

Posted - Jan 23 2004 :  00:29:07  Show Profile Send Paul B a Private Message  Reply with Quote
Theoretically, a 1 in 12" twist will stabilize up to a 200 gr. bullet. That's theoretically. It'sd also balony! I have two 30 caliber rifles with 1 in 12" twists that I've tried 220 gr. bullets in, oner a 30-06, the other a .308 Win.
What brought some of this up, for me, was one of the "egg-spurts" in one of the better gun rags said that the heaviest bullet one could use in a 30-06 with 1 in 12" twist was the 180 gr. I wrote the "gentleman" a letter quoting the late Jack O'Connor, no less as having debunked that theory years ago, and to back it up, sent targets shot with my 30-06 with 1 in 12" twist to prove the point.
I even challenged him to stop by, if he was ever in town, and he could check out the rifle, the bulets and the load for himself, rather than just take my word for what I had said.
I got back a nice letter saying that he stood corrected, but even though he would be in Tucson for a dove shoot, there was no way he could meet with me and settle the subject. (His majesty was too damn busy.)
About six months later, he writes an article on the "myths" in gun shooting. Nice article where he, for all practical purposes, calls me a liar. As the magazine's headquarters are only about a three hour drive for me, I suppose I should have gone on up and represented my challenge by presenting the rifle, bullets, loads and the letters involved. It still rankles me a bit, but that gun rag used to take articles from unknowns and publish them. Now that they've hire on all the (expletives deleted) that were dropped from Guns & Ammo, they can no longer afford to take free lance writings. Too bad as their magazines have taken a long downhill turn.
Another myth that proves to be wrong is a .308 Win. will shoot 220 gr. bullets to a decent velocity. If a 30-40 Krag ias good at 2000 to 2200 FPS, why isn't the .308 at 2300 just as good, if not better.
FWIW, the 30-06 gave groups in the 1.25"-1.50" range, and the M70 Winchester with 1 in 12" in .308 gave groups of .375" to .50"
Guess you just can't believe what those gun writers write, can you?
Paul B.
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Sabot
Junior Member

46 Posts

Posted - Jan 26 2004 :  18:49:48  Show Profile Send Sabot a Private Message  Reply with Quote
Of course, this is the famous Greenhill formula, which can be simplified as bore squared times 150 (or 180) equals twist required for a 1 inch long bullet. Then you just scale your length up or down proportionately to increase or decrease twist rate, respectively.

So, .308X.308X150 = 14.22 inch twist rate for 1 inch bullets.

A 2 inch long .308 would need half that, or a 7.1 inch twist rate.

To turn the math around, a 12 inch twist rate will stabilize a 1.19 inch long .308 bullet. However, this is an OPTIMAL twist rate, and a 1.3 inch long .308 will also stabilize with acceptable accuracy.

If you are shooting around 2700 fps, this formula is quite accurate, but as you slow down or speed up it is less predictive. Lower velocities require faster twist and higher velocities need a slower twist. A 20 % increase or decrease changes the twist rate by about 10% up or down. This is why you sometimes see a Greenhill constant of 180 instead of 150...it applies to faster rifles used today. At 2300 fps, a 1.2 inch long .308 bullet would be at the threshold of instability.
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