Twist Rate
- Thread starter Hgold
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Re: Twist Rate
Hgold,
For a bullet to fly point-forward, it must spin fast enough to be stable. What factors determine stability?
Caliber
Bullet weight
Bullet length
Muzzle Velocity
Barrel Twist
Temperature
Atmospheric Pressure
It is a combination of those 7 factors that affect bullet stability, not just barrel length. But of those factors bullet length has the most immediate influence.
So it's not a question of short barrel and fast twist VS long barrel and slow twist, but more a function of long bullet fast twist, and short bullet slow twist.
To actually calculate what twist and speed you need to stabilize a particular bullet go here:
JBM - Miller Stability calculator
Hgold,
For a bullet to fly point-forward, it must spin fast enough to be stable. What factors determine stability?
Caliber
Bullet weight
Bullet length
Muzzle Velocity
Barrel Twist
Temperature
Atmospheric Pressure
It is a combination of those 7 factors that affect bullet stability, not just barrel length. But of those factors bullet length has the most immediate influence.
So it's not a question of short barrel and fast twist VS long barrel and slow twist, but more a function of long bullet fast twist, and short bullet slow twist.
To actually calculate what twist and speed you need to stabilize a particular bullet go here:
JBM - Miller Stability calculator
Re: Twist Rate
Yip. Two separate questions:
Twist rate: what projectile do you plan on firing, where, and how fast?
Barrel length: do you need a more wieldy rifle? How much velocity will you lose with a shorter barrel? Is that velocity loss acceptable?
An example is that I have a 10" twist 308. In hindsight, I wish I had gone for an 11" twist. Why? I rationalized going with a 10" twist to allow me to stabilize the heavies, but the only projectiles that the 10" will stabilize that an 11" won't is in the 220 - 240 gr range. If I were to shoot projectiles that heavy with the way that my rifles is floated, the bullet would be robbing all the room for the powder, so I am not using these bullets anyway. Of course, the 10" is handy for shooting subsonics, but that is kind of gimmicky. It is a feature I can live without.
Bear in mind when you select a twist rate that will faster twist will stabilize better, the faster twist also robs accuracy as any "imbalance" within the bullet will be magnified. So as far as twist rate goes, just good enough to stabilize what you want to shoot is the right answer.
On barrel length, certain cartridges need the length to generate the appropriate velocities. I shoot a 308 and with my 22" barrel, I am getting similar velocities to people I know shooting 26" barrels. The 308 is not that sensitive to barrel length until you get down to 20" and less. There is a slight drop off from 20" to 18" and then a pretty big one once you go down to 16" or so. Different calibers behave differently. Based on what you want to do with the rifle and which caliber you select, your answer might be different than someone else's. You can do some research and there are ballistic programs available to help you determine the tradeoff that works for you.
Yip. Two separate questions:
Twist rate: what projectile do you plan on firing, where, and how fast?
Barrel length: do you need a more wieldy rifle? How much velocity will you lose with a shorter barrel? Is that velocity loss acceptable?
An example is that I have a 10" twist 308. In hindsight, I wish I had gone for an 11" twist. Why? I rationalized going with a 10" twist to allow me to stabilize the heavies, but the only projectiles that the 10" will stabilize that an 11" won't is in the 220 - 240 gr range. If I were to shoot projectiles that heavy with the way that my rifles is floated, the bullet would be robbing all the room for the powder, so I am not using these bullets anyway. Of course, the 10" is handy for shooting subsonics, but that is kind of gimmicky. It is a feature I can live without.
Bear in mind when you select a twist rate that will faster twist will stabilize better, the faster twist also robs accuracy as any "imbalance" within the bullet will be magnified. So as far as twist rate goes, just good enough to stabilize what you want to shoot is the right answer.
On barrel length, certain cartridges need the length to generate the appropriate velocities. I shoot a 308 and with my 22" barrel, I am getting similar velocities to people I know shooting 26" barrels. The 308 is not that sensitive to barrel length until you get down to 20" and less. There is a slight drop off from 20" to 18" and then a pretty big one once you go down to 16" or so. Different calibers behave differently. Based on what you want to do with the rifle and which caliber you select, your answer might be different than someone else's. You can do some research and there are ballistic programs available to help you determine the tradeoff that works for you.
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