308 v 223

[woollyback 0]

I know people are going to think I'm completely daft asking this question, but a shooting friend (non rifle shooting) and I were talking about my rifles the other day and he asked me how a 223 and a 308 can have the same 1:12 twist rate and shoot completely different rounds - my 223 shooting 55gr and my 308 150-180gr. I wasnt sure how to answer his question correctly; But i said I new a place where his question would get a professional answer.

[Mr_Logic 5]

There is a formula which I can't be bothered to dig out right now, but it's all to do with calibre. Different width of bullet means different lengths are stabilised by the same twist. Other thing to bear in mind is velocity. For example, a 1:14 22-250 will stabilise broadly the same bullet lengths as a 1:12 223 because the 22-250 drives the bullet faster and generates more rpm.

[quicksilver 0]

i read a good article in last months shooting sports which explained the GReenhill Formula and how it is used to work out the twist rate needed to stabilise certain bullets. It mainly deals with the bullet length as oppossed to bullet weight but as it has been pointed out previously heavier bullets of the same caliber will be longer.

 

an example in the article relates to 168G BTHP in .308

 

length in calibers = length of bullet / caliber = 1.213" / .308 = 3.938 calibers

therefore 150 / 3.938 = 38.09

to turn this into the twist rate required to stabilize the above bullet you need to mulitply 38.09 x .308 ( caliber in inches) = 11.73

or a rifling twist rate of 1:12

[woollyback 0]

Yes I gave him a basic explanation but didnt know the technical stuff. cheers fellas!