Help me understand rifle groove spin length in a M1 carbine vs 1903.

[Karen]

My Rockola grooves turn one rotation every 20 inches. The little 110 bullet/boolit run at 2000 fps +-
I can load my Dad's 1903 with the same 110 bullets but they rotate twice as fast, 1 cycle per 10 inch.

Remington 1903 30-06 bullets are 220, 174 or 150 grain in the same 10 inch rotation and are stable at long distance. That seems to be a big range of bullet weights at 1 - 10 cycle rate.
If I could fire a 150gr or 174gr bullet in my carbine at 1 in 20 turn at 2000fps, would it be stable? How about 2500fps?
So... it seems to me that a bullet/boolit that is long or heavy needs to turn very fast vs small/short bullets? How is rifle groove turn rate determined?

PS -my major was Biology, not math.

[Mooseman]

Your deduction is correct.
Longer bullets (which are heavier) need a faster twist rate to have a stabilized flight.
Shorter, lighter bullets will stabilize with less twist.
Rifle makers know what the average bullet weight is that is available in factory ammo and rifle the twist accordingly to a happy medium.
For instance My 1:9 twist 300 magnum will stabilize from 110 to 220 gr Bullets.
180 and 220gr. were the common available load.

Rich

[Karen]

Your deduction is correct.
Longer bullets (which are heavier) need a faster twist rate to have a stabilized flight.
Shorter, lighter bullets will stabilize with less twist.
Rifle makers know what the average bullet weight is that is available in factory ammo and rifle the twist accordingly to a happy medium.
For instance My 1:9 twist 300 magnum will stabilize from 110 to 220 gr Bullets.
180 and 220gr. were the common available load.

Rich

Does the velocity of the bullet matter? Like 2000fps ve 3000fps?

[Norbrat]

Does the velocity of the bullet matter? Like 2000fps ve 3000fps?

Yes, it can.

http://en.wikipedia.org/wiki/Rifling

The section on Greenhills Formula, etc.

[Karen]

Yes, it can.

http://en.wikipedia.org/wiki/Rifling

The section on Greenhills Formula, etc.


That article is way too complicated for me.
I suppose my thought is this, with a 30-06 and its twist of 1-10, you can use just about any bullet and it will perform well? Because of the fast twist? Maybe that is why the 30-06 is so popular?

But if you use a slow twist like 1-20, larger bullets won't do well?
What if the bullet is going very slow, like a black powder rifle? Does it need a fast twist rate still? say a 150 or 180 grain bullet?

Trying to understand.

[Norbrat]

The Greenhills Formula basically says that the length of the bullet relative to bore diameter is a major factor in determining twist. The velocity seems to be a less important factor, but it is still a factor once the velocity is higher than about 2800 fps.

So logically a heavier bullet is usually longer, therefore it requires a faster twist. Of course bullet design; ie, hollow pointing and nose shape, will result in a lighter bullet of similar length.

Black powder rifles, if using round balls which are very short relative to the diameter of the bore, will stabilise the ball with a slow twist. Accuracy of longer minie style bullets would very likely be improved with faster twist.

[Larry Gibson]

The longer a bullet is per given caliber (diameter) the faster the rate of spin (twist) is required to minimally stabilize it for accurate flight. Thus, per a given caliber the heavier the bullet is the longer it will be and the faster the twist must be.

The 18 - 20" twist is optimum for the 107 gr jacketed RNs at 1950 fps out of the Carbine. The same bullet at the same velocity out of the '06 with a 10" twist is way over stabilized. A 220 gr RN is very stable out of the 10" twist at 2000 fps but would be unstable at 2000 fps out of the 20" twist of the 30 Carbine (if you could drive it that fast).

If I could fire a 150gr or 174gr bullet in my carbine at 1 in 20 turn at 2000fps, would it be stable? How about 2500fps?

No for the 174 gr but the 150, depending on the bullets design/length may be minimally stable at 2500 fps.

Larry Gibson