Barrel twist vs caliber

M

model14

Guest
Why does a 180 grain .308 like a 1:13 twist but a 180 grain .284 likes 1:9 twist?

Or better yet, a 180 grain .308 and a 40 grain .223 both shoot 1:13 well.

A 180 grain 30-06 has a 1:10 as standard, as does a 130 grain .270.

Is there a rhyme or reason involved here?
 
MRL said:
The twist required for a given bullet is not determined by the bullet weight but by the overall length of the bullet. Shorter bullets need less twist than long bullets for a given velocity.
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You knew this was coming: why? :confused:
 
Because a longer bullet has a larger overturning moment, and must be spun faster to prevent tumbling. The overturning moment is proportional to the distance between the bullet's center of mass and its center of pressure. In a longer bullet the center of mass moves further towards the base, while the center of pressure doesn't move much.

Toby Bradshaw
baywingdb@comcast.net
 
bsl135 said:
model14,
You've got some good questions!
The answer to this one is long, but fortunately it's already written:
http://www.appliedballisticsllc.com/index_files/Understanding_part1.pdf

I recommend reading the whole thing, but if you just want to skip to the bit about 'scaling effects on stability', that starts around page 7.

-Bryan
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Bryan,

Not only have I read the whole thing, but I am busy copying your other articles also for some good study. I still enjoy and absorb better when looking at hard copy instead of the computer monitor. Thanks for all of the great information. Berger is lucky to have your services.

Richard
 
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