Bullet RPM and performance

[jasent]

Berger
185's and 210's from 1:8

 

[Grumpy old man]

Berger
185's and 210's from 1:8

What powder h1000?

 

[jasent]

Yes

 

[Calamity]

Hopefully I am not beating a dead horse here or asking an ignorant question.

I have read about twist in relation to bullet stability and velocity in relation to bullet performance at long range. I just can't recall about reading if RPM contributes to bullet performance (i.e mushrooming and transfer of energy).

I know we have all read instances of penciling, the ELD-X bullets are one I experienced first hand and at a variety of ranges.

My questions are:

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Is there a way to know you are in the sweet spot in relation to twist and bullet/load selection other than accuracy at the range?

I am in the process of deciding on a new barrel and other than stability for the chosen bullets, I am wondering what other effects twist will play with my selection.

Thanks Darryle

Physics texts refer to spin as angular velocity, and describe what we generally think of as centrifugal force (force trying to shed the bullet jacket) as centripetal force. It's just semantics, but I mention it in case you want to dig into it, and find the nomenclature confusing. Given that rotational speed (RPM) is determined by twist rate and velocity, the centripetal force is further determined by the diameter/radius of the bullet. Given a certain rotational speed, larger diameter bullets exert more force on the bullet jacket than slimmer diameter bullets, so a .44 bullet requires less rotational speed than a 6mm bullet, so you can't just pick an RPM number that is good for all calibers. Bullet designers can compensate for a range of RPMs by making jackets thicker, or using a stronger alloy like steel with copper plating. The bullet makers have taken these things into consideration for you, so using their calculators to check the range of bullet weights (their bullets) you intend to use for your caliber/velocity should dictate the most likely twist you need in a barrel. I hope this is helpful.

 

[Radman]

Physics texts refer to spin as angular velocity, and describe what we generally think of as centrifugal force (force trying to shed the bullet jacket) as centripetal force. It's just semantics, but I mention it in case you want to dig into it, and find the nomenclature confusing. Given that rotational speed (RPM) is determined by twist rate and velocity, the centripetal force is further determined by the diameter/radius of the bullet. Given a certain rotational speed, larger diameter bullets exert more force on the bullet jacket than slimmer diameter bullets, so a .44 bullet requires less rotational speed than a 6mm bullet, so you can't just pick an RPM number that is good for all calibers. Bullet designers can compensate for a range of RPMs by making jackets thicker, or using a stronger alloy like steel with copper plating. The bullet makers have taken these things into consideration for you, so using their calculators to check the range of bullet weights (their bullets) you intend to use for your caliber/velocity should dictate the most likely twist you need in a barrel. I hope this is helpful.

Well said!

 

[436]

Hopefully I am not beating a dead horse here or asking an ignorant question.

I have read about twist in relation to bullet stability and velocity in relation to bullet performance at long range. I just can't recall about reading if RPM contributes to bullet performance (i.e mushrooming and transfer of energy).

I know we have all read instances of penciling, the ELD-X bullets are one I experienced first hand and at a variety of ranges.

My questions are:

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Is there a way to know you are in the sweet spot in relation to twist and bullet/load selection other than accuracy at the range?

I am in the process of deciding on a new barrel and other than stability for the chosen bullets, I am wondering what other effects twist will play with my selection.

Thanks Darryle

You might pick up P.O Ackley book(s) volumes 1 & 2 he speaks a lot about your question(s). Granted it's an older read, but... not much in most physics questions relating to ballistics has changed over the years. Good luck... Cheers.

 

[nralifer]

Bullet RPM's are the result of the combination of twist rate and muzzle velocity. A heavier bullet fired from the same case will likely have less velocity, and therefore may require a faster twist rate to achieve sufficient RPM's to be stable. For the same reason a lighter bullet fired from the same case will tend to have higher velocity thus the fast twist rate may create too many RPM's resulting in deforming or disintegration. Any bullet is going to have a velocity range that give optimum terminal performance on impact which will be based on the construction and materials of that bullet. Seems to me that optimum terminal performance will depend on impact velocity and whether that bullet is stable at that velocity. That is what will give best mushrooming and RPM has little to do with that other than providing the stability since the RPM will go to zero almost instantly on impact.

Our gel tests show the "wound channel" is spiral shaped throughout its entire length. It's clear that the bullet spin is persisting throughout the full length of the wound channel. These are reduced velocity loads, usually 2000 fps or less, so the rpm of the bullet is less than it would have at 3000 fps.