Velocity vs. Barrel Twist Rates

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Did what you suggested and found a very good article explaining twist rates.
Seems to me that a slower twist rate would in fact possibly increase velocity due to less friction and add stability to heavier slugs.
Please correct me if I'm incorrect in my conclusion.
I'm bought a slug liner for my .25 600mm crown mk2. Currently I'm shooting the standard liner in it 29 gr. slugs at 975fps. Accuracy is good , averaging 1/2" 5 shot groups at 57yards due to occasional flier. I'm hope to reduce that average group size to 3/8 to 1 hole.
Thank you for your suggestion.
 
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Slugs are spin stabilized and (by and large) larger/heavier slugs require more spin (rpm). Spin is increased in barrels with a faster twist rate. Energy pushing the projectile contributes to both muzzle velocity and spin, so if you increase spin, you will reduce muzzle velocity. However, the slug/heavy liners are not just different in terms of spin. I don’t know exactly how your liners may differ in other ways but if the new liner has less choke for example, that could offset the negative impact of increased spin on muzzle velocity. There are many factors that determine the accuracy (and persistence of accuracy) shooting slugs. Different instances of the same liner can differ markedly with the same slug and tune. Also, you may find that your replacement liner “prefers” a different slug or tune.

Your group size is already relatively good and fliers can occur for many reasons. A lot of hype abounds but very few people/guns can shoot MOA consistently.
 
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A lower twist rate will never "add stability", particularly if the muzzle velocity increases. Gyroscopic stability increases with the square of the ratio between the spin rate and the forward velocity. At the muzzle, the spin velocity ratio is given by the twist rate. If you look at any of the bullet stability calculators, you will see that as velocity increases you need to increase the twist rate for stability, not reduce it.

There are circumstances where, if your slugs have too much stability, group sizes can be reduced by reducing twist rate and thus reducing stability. A twist rate which gives a stability factor of 1.5 has been shown to be optimum for BC, but may not be optimum for group size. However, there are many other factors which are likely to have a much bigger effect on your group size than twist rate, providing it is not stupidly far away from the optimum.
 
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Ballisticboy said:
A lower twist rate will never "add stability", particularly if the muzzle velocity increases. Gyroscopic stability increases with the square of the ratio between the spin rate and the forward velocity. At the muzzle, the spin velocity ratio is given by the twist rate. If you look at any of the bullet stability calculators, you will see that as velocity increases you need to increase the twist rate for stability, not reduce it.

There are circumstances where, if your slugs have too much stability, group sizes can be reduced by reducing twist rate and thus reducing stability. A twist rate which gives a stability factor of 1.5 has been shown to be optimum for BC, but may not be optimum for group size. However, there are many other factors which are likely to have a much bigger effect on your group size than twist rate, providing it is not stupidly far away from the optimum.
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@ Ballisticboy

Assuming this advice refers only to slugs, is there any equivalent information for pellets ?
 
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Ballisticboy said:
A lower twist rate will never "add stability", particularly if the muzzle velocity increases. Gyroscopic stability increases with the square of the ratio between the spin rate and the forward velocity. At the muzzle, the spin velocity ratio is given by the twist rate. If you look at any of the bullet stability calculators, you will see that as velocity increases you need to increase the twist rate for stability, not reduce it.

There are circumstances where, if your slugs have too much stability, group sizes can be reduced by reducing twist rate and thus reducing stability. A twist rate which gives a stability factor of 1.5 has been shown to be optimum for BC, but may not be optimum for group size. However, there are many other factors which are likely to have a much bigger effect on your group size than twist rate, providing it is not stupidly far away from the optimum.
Click to expand...
I see your point. Thank-you
 
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Bluestone said:
@ Ballisticboy

Assuming this advice refers only to slugs, is there any equivalent information for pellets ?
Click to expand...

Pellets get more gyroscopically stable as speeds begin to approach Mach 1 (1116.5 ft/sec) as they begin to lose aerodynamic stability. At some point, depending on the pellet shape and mass distribution, it can have infinite gyroscopic stability due to its aerodynamic stability coefficient being zero, which is going to be bad for group sizes as the pellet cannot correct for yaw and cannot turn to follow the trajectory. As the pellet slows down it will reduce its gyroscopic stability which will enable it to start correcting for yaw, but it will be too late for accuracy.

I did produce a lot of modelling data looking at how twist rates affect group sizes for the JSB 8.44 and 15.9 grain pellets. The best twist rates seemed to lie in the range from 15 to 50 inches per turn for both pellets, but that was at sub 12 FPE and ranges up to 50 yards. For different pellet designs, it could well be different. I have not done it for any other design of pellet as it is very difficult to get the calibrated data needed for such exercises, even for the two pellets above not all the input data was verified, though the important variables were. The modelling itself is also time intensive, needing hundreds of trajectory runs. It may be possible to carry out modelling for JSB Heavies, as I have estimated data for the .177 version. Even if the data is not verified, it could still show up if there are significant differences in the ideal twist rates for the different pellet designs.
 
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weevil said:
Slugs are spin stabilized and (by and large) larger/heavier slugs require more spin (rpm). Spin is increased in barrels with a faster twist rate. Energy pushing the projectile contributes to both muzzle velocity and spin, so if you increase spin, you will reduce muzzle velocity. However, the slug/heavy liners are not just different in terms of spin. I don’t know exactly how your liners may differ in other ways but if the new liner has less choke for example, that could offset the negative impact of increased spin on muzzle velocity. There are many factors that determine the accuracy (and persistence of accuracy) shooting slugs. Different instances of the same liner can differ markedly with the same slug and tune. Also, you may find that your replacement liner “prefers” a different slug or tune.

Your group size is already relatively good and fliers can occur for many reasons. A lot of hype abounds but very few people/guns can shoot MOA consistently.
Click to expand...
Thanks
 
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