Why do heavier bullets need shorter twist rate

ckc123

ckc123

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I understand the issues around finding a good twist rate, and that the heavier bullets require a shorter twist (faster/tighter).. (and the too short will wear out the barrel faster..)

but why? is it because of the actual weight, or the slower velocity or other factors?
 
ckc123 said:
I understand the issues around finding a good twist rate, and that the heavier bullets require a shorter twist (faster/tighter).. (and the too short will wear out the barrel faster..)

but why? is it because of the actual weight, or the slower velocity or other factors?
Click to expand...

It is because of the length of the bullet. A long bullet requires more spin to keep it stabilized ... to keep it from tumbling.

Compare it to a base ball and a football... the football will fly through the air much better with spin to it... watch those long bombs... a very nice spin to them..

The barrel does not wear because of the twist, it's what being shot in it that does it.

Get your twist rate from any barrel maker, they have recommendations to match the bullet ...
 
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guntech said:
It is because of the length of the bullet. A long bullet requires more spin to keep it stabilized ... to keep it from tumbling.

Compare it to a base ball and a football... the football will fly through the air much better with spin to it... watch those long bombs... a very nice spin to them..
Click to expand...

Ahh.. ok makes sense.. so in theory. if the material of the bullet was made to be more dense then lead, then you would not need to increase the twist for the heavier bullet..
 
ckc123 said:
Ahh.. ok makes sense.. so in theory. if the material of the bullet was made to be more dense then lead, then you would not need to increase the twist for the heavier bullet..
Click to expand...

No that is not correct. The Greenhill formula for twist rate was developed in the late 1800s for artillery projectiles but is applicable to small projectiles as well. When you go to dramatically heavier or lighter materials like depleted uranium or aluminum there is a correction factor although I don't have it off the top of my head. Also the formula takes into account both length and diameter so that a 1" long slug in .22 cal does not take the same twist as a 1" slug in .50 cal. Increasing velocity will slightly increase stability but very little. Finally the rotational speed of a bullet seems to slow somewhat less than its forward velocity otherwise you would find bullets becoming rapidly unstable with distance from the gun.
the formula by the way is 150 x diameter squared over length equals twist rate although most people would make the twist about 1" faster.

cheers mooncoon
 
ckc123 said:
Ahh.. ok makes sense.. so in theory. if the material of the bullet was made to be more dense then lead, then you would not need to increase the twist for the heavier bullet..
Click to expand...

Not entirely - there are other factors involved, but yes, it is the length that is the big issue. I thump out 220 grain Hornady round-nose bullets out of one of my 308's (1:12 twist), and in that particular gun, they are the most accurate bullet I've ever shot in it. The Round Nose bullet is very short - as I recall, it's actually shorter than a 180 grain Ballistic Tip - and this is why this works.


There's a ballistic calculator out there somewhere you can use to calculate stabilization factor, I'll go see if I can find a link for you.
 
Ok makes even more sense now.. I was reading about AR15s and the twist rates, and I wanted to understand the dynamics of the twist, not just the recommendations people where making..

Thanks!!!!
 
ckc123 said:
Ahh.. ok makes sense.. so in theory. if the material of the bullet was made to be more dense then lead, then you would not need to increase the twist for the heavier bullet..
Click to expand...

Correct. In fact, with this denser, shorter bullet of the same weight, you could use a barrel with a slower twist rate, and still be stabilized.

A good starting point to learn and understand about bullet stabilization is "Greenhill's Formula". Look it up for more details, but here's what it says:

Length * twist < 150 for a stabilized bullet

(fine print: "Length" is the length of the bullet, measured in calibers. "Twist" is the twist rate of the barrel, measured in calibers. So a .308" diameter (caliber) bullet that is 1.00" long, would have a length of 1.00"/0.308" = 3.25 calibers. A 1 in 12" twist barrel would have a twist rate of 12"/.308" = 38.97 calibers. So length*twist = 3.25*38.97 = 126.6. Since this is less than 150, then this is stable according to Greenhill)

Disclaimer: Greenhill is a good starting point. There is more to it that that, and a lot more refinements to what affects whether or not a give bullet and twist combination will be stable, but it is a good place to start.
 
And just to add, twists substantially faster than is needed to stabilize a particular bullet does not contribute to accuracy.

Say in .224 caliber, if you are only ever going to shoot bullets lighter than 55 grain, a 1:14 twist will do just fine.

Original M16 rifles had a 1:14 twist.
 
.... and twists slightly faster do not detract from accuracy...

... it is only something that may affect heavy serious short range benchrest rifles...

give me a 12 twist for the 22 caliber long 55's ...
 
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