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Post by whyohe on Jan 24, 2009 7:51:07 GMT -5
ok we talked about before the longer the bullet the faster the twist to stabilize and for greater accuracy. now the question is there a formula or ratio to get close to chosing a bullet for your gun? example: a xx length to xx lenght bullet for a x:xx twist.
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Post by Dave W on Jan 24, 2009 9:16:11 GMT -5
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Post by whyohe on Jan 24, 2009 21:21:42 GMT -5
thanks that is some very useful info.
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Post by tcmech on Jan 31, 2009 21:11:50 GMT -5
There is a formula which states the length of the bullet (in caliber) will determine the optimum twist for a barrel. I don't really know how to explain it but it is determined by bullet length and caliber. In other words a bullet that was .9" fired from a 30 caliber bore would be 3 calibers long, a buller .9" fired from a 45 caliber bore would be 2 calibers long. Try to google "greenhill formula" and see what hits you get.
Craig
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Post by edge on Feb 3, 2009 9:36:20 GMT -5
Greenhill is indeed useful, but it usually yields a stability factor of 1.5 ( 150% of the minimum ).
You can almost always go with less twist than Greenhill calls out, but you need to guard against "local" conditions!
Summer shooting requires much less twist than Winter due to air density. High pressure and low temperatures require more twist, but Greenhill takes the worst cases into account.
Greenhill also has an abrupt change at 2800 fps for required twist.
edge.
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Post by edge on Feb 3, 2009 10:22:39 GMT -5
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Post by whyohe on Feb 3, 2009 17:38:37 GMT -5
WOW i'd never taken all that into consideration. thanks edge
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