Twist rate versus barrel length question

[RSDesertStorm]

I have an unfired Colt HBAR A3 and have a question in regards to its twist rate. It has a 9 twist rate but the barrel length is 24” long.

Does the extra length add spin rate over a 9 twist barrel say 16 inches? This is assuming higher bullet velocities increase spin rate.

And if yes what is the heaviest bullet I can put in there and maintain stable spin out to 1000 yards? I‘m looking to get the highest BC out of my ammo that one day I hope to build a load to. Tnx.

 

[XLR308]

I have a 1-9 twist 16" chrome lined that shoots everything up to 75bthp just fine with moderate loads but it definitely shoots 50-65 grain bullets best.
I think it will come down to what the barrel likes best but believe what you want to do is possible just not ideal.
Have you played around with the stability calculater on Bergers site ?

 

[Molon]

I have an unfired Colt HBAR A3 and have a question in regards to its twist rate. It has a 9 twist rate but the barrel length is 24” long.

Does the extra length add spin rate over a 9 twist barrel say 16 inches? This is assuming higher bullet velocities increase spin rate.

While the increased RPM due to the faster velocity produced by a longer barrel will act to increase the bullet stability, the increased velocity also

“increases the force applied to the nose of the bullet at the center of pressure and strengthens the overturning aerodynamic torque which actually makes the bullet less stable.”*

These two opposing dynamics are the reason for the miniscule increase in gyroscopic stability as barrel length increases.



* From Applied Ballistcs For Long-Range Shooting by Bryan Litz



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[RSDesertStorm]

Thank you for that informative lesson. it sounds like there’s a trade off for the increased spin, a higher frontal pressure offsets the stabilization benefits of increased spin rotation.