*New Results* Forward Velocity vs Bullet RPM....which “hits” harder?

[AllThingsAI]

I’ll throw this in the mix. Watch this one.

I love Matt’s videos...I watch all of them. He really understands ballistics harmonics, and how to shoot. Great video...a lot of fun to ponder what is Really going on.

BTW if you really want to learn how to shoot rainbows, hold over, and adjust for the wind, get into PCP air rifles. They are amazingly accurate and powerful.

 

[markT]

Love pcps. Been shooting them for years and have killed hundreds of squirrels and sparrows. Nothing like it.

 

[AckleymanII]

I can not answer your question exactly but I can share my experience with twist. I have shot varmint with the 6br,dasher,bra cases with zero free bore barrels from 7.75 to 13.5tw. I will never do another 13.5. The fast twists seem to be about twice as explosive on target. A couple hundred feet a second does not really have an obvious difference to me.

I tried fast twists back in the 80's on p. dogs.

I had two conclusions:

a. if you want bullets to blow up from Kinetic energy(rpm) seeing a major difference in impact, you have to use a very fast twist
7 twist in 22 caliber was exceptional with tough 52-55g bullets at 2900 fps, which bested the explosiveness of a tough 55g bullet at 3550! 40g Nosler ballistic tips blew up better with 9T vs 14 twists.

B. fast twists tightened the accuracy envelope in terms of a spread of powder charges that would be super accurate. In practical terms, this had an effect on head shooting p. dogs at 300 as they would lay in their holes with nothing but their heads stuck out of the holes.

I had a 22 PPC set up with a Hart 9T, and loads got very sensitive, 223 with a 7 Twist, and a 6 PPC set up with a Hart 8T.

If you want to see more Helicopters and twisted up dogs, go real fast on your twist choices, not just a little faster. 8T in both 22 and 6mm will get the job done in spades, while 10 twists will not...tried that also. Be sure to match your freebore length with the bullets you are shooting....critical!

For hunting coyotes, I did not want surface explosions, so the slowest twist to match the bullet length worked the best for penetration and breaking bones.

I have wanted to hit a coyote with a 90g tnt out of a 8T 6.5x47 Lapua, should kill very humainly!

 

[AllThingsAI]

I tried fast twists back in the 80's on p. dogs.

I had two conclusions:

a. if you want bullets to blow up from Kinetic energy(rpm) seeing a major difference in impact, you have to use a very fast twist
7 twist in 22 caliber was exceptional with tough 52-55g bullets at 2900 fps, which bested the explosiveness of a tough 55g bullet at 3550! 40g Nosler ballistic tips blew up better with 9T vs 14 twists.

B. fast twists tightened the accuracy envelope in terms of a spread of powder charges that would be super accurate. In practical terms, this had an effect on head shooting p. dogs at 300 as they would lay in their holes with nothing but their heads stuck out of the holes.

I had a 22 PPC set up with a Hart 9T, and loads got very sensitive, 223 with a 7 Twist, and a 6 PPC set up with a Hart 8T.

If you want to see more Helicopters and twisted up dogs, go real fast on your twist choices, not just a little faster. 8T in both 22 and 6mm will get the job done in spades, while 10 twists will not...tried that also. Be sure to match your freebore length with the bullets you are shooting....critical!

For hunting coyotes, I did not want surface explosions, so the slowest twist to match the bullet length worked the best for penetration and breaking bones.

I have wanted to hit a coyote with a 90g tnt out of a 8T 6.5x47 Lapua, should kill very humainly!

Thanks for the info, good stuff...I want to quantify this affect a bit more. I decided on the following target setup

12x12 frame:
Layer:
1. Rabbit fur stretched out
2. 1/4” air gap
3. Construction paper
4. 1/4” air gap
5. Construction paper
Repeat until I have 4” thick target

This should show me the 3D fragmentation “cone” after first impact.
Will shoot at 100 yards, 6BRs at the same velocity (tbd) 1:16 barrel vs 1:8, 55 grain Sierra Blitzking for first proof of concept.

Hypothesis is the cone will expand wider through trash layer with the higher twist. We shall see!

 

[mensajd]

For normal varmint cartridges at normal velocities and twist rates the translational kinetic energy component dominates the rotational kinetic energy component. It’s a trivial calculation and the proof is left as an exercise for the reader. Yes I was a theoretical math major.

 

[AllThingsAI]

For normal varmint cartridges at normal velocities and twist rates the translational kinetic energy component dominates the rotational kinetic energy component. It’s a trivial calculation and the proof is left as an exercise for the reader. Yes I was a theoretical math major.

Do you happen to use mathcad/matlab? I would love to see some of your energy calculations when the two forces are combined and if material properties of different bullets could be modeled (That would be amazing). But for a bullet hitting “hard” is it’s not a matter of one energy dominating vs the other, but of how quickly that rotational energy spreads fragments out after impact and the forward energy gets spread over a larger area....therefore more energy transfer in a shorter distance. mathematically this is very trivial as well.

My hypothesis is this fragmentation cone will be larger and expand faster in a higher twist barrel for a given bullet and velocity.

 

[mensajd]

Do you happen to use mathcad/matlab? I would love to see some of your energy calculations when the two forces are combined and if material properties of different bullets could be modeled (That would be amazing). But for a bullet hitting “hard” is it’s not a matter of one energy dominating vs the other, but of how quickly that rotational energy spreads fragments out after impact and the forward energy gets spread over a larger area....therefore more energy transfer in a shorter distance. mathematically this is very trivial as well.

My hypothesis is this fragmentation cone will be larger and expand faster in a higher twist barrel for a given bullet and velocity.

I’m in tech but no longer do numerical analysis. But the translational KE doesn’t require that. The rotational KE is more complex but you can make some simplifying assumptions and just assume that the bullet is a cylinder of uniform density. Approximately worst case. Or you can model the shape of the jacket and core using piecewise differentiable functions, then piecewise integrate and figure out a good model for the bullet I (rotational moment) but the results don’t change much relative to translational KE vs rotational KE.

 

[AllThingsAI]

I’m in tech but no longer do numerical analysis. But the translational KE doesn’t require that. The rotational KE is more complex but you can make some simplifying assumptions and can just assume that the bullet is a cylinder of uniform density. Approximately worst case. Or you can model the shape of the jacket and core using piecewise differentiable functions, then piecewise integrate and figure out a good model for the bullet I (moment) but the results don’t change much relative to translational KE vs rotational KE.

When the material properties are factored in the complexity goes up exponentially. Distributed forces along a rotating bullet of a given construction is where the material scientists come in and I lose my steam. I’m getting too old for differential equations!

I did calculate the total energy for both forward motion and rotation using a 55g 6mm bullet

Forward: 1490J @ 914.4m/s (3000fps)
2648J @ 1219m/s (4000fps)

Rotational: 4.19J @ 150,000RPM
16.75J @ 300,000RPM

So you are 100% correct that the forward kinetic energy dominates the rotational....but the rotational energy will send fragments out radially with 4x the force (when RPM doubles) once impact occurs. So the Fragment dispersion cone should be greater and dump more of that forward kinetic energy over a larger area.

Had to dust off the calculator! Feel free to check my math, the conversions can always bite you.

 

[maxwatt]

In my eyes this thread is the most interesting in a loong time. But I feel it took a wrong turn initially.
In stead of thinking about the difference between exploding bullets and more explosive bullets, why not try to think about the effects of spin on solid copper expanding bullets.
They generally hold together after impact and so they can more easily show quantifiable differences in expansion.
It is well known that you have to have enough velocity on impact with these bullet types to ensure the kinetic energy is going to expand it. So a bullet shot at a too long range expands less and can pencil through game.
Rotational or centrifugal forces and the effect it has on expansion of copper bullets is something I feel has not been explored and "investigated" fully by the shooting community.
For example a 150gr. 30Cal from a 12 or 14 twist at a velocity in the lower ranges and the same bullet from a much faster twist and same velocity shot in ballistic gel should show clear difference in expansion if rotational energy has a significant contribution to bullet expansion.

 

[AllThingsAI]

In my eyes this thread is the most interesting in a loong time. But I feel it took a wrong turn initially.
In stead of thinking about the difference between exploding bullets and more explosive bullets, why not try to think about the effects of spin on solid copper expanding bullets.
They generally hold together after impact and so they can more easily show quantifiable differences in expansion.
It is well known that you have to have enough velocity on impact with these bullet types to ensure the kinetic energy is going to expand it. So a bullet shot at a too long range expands less and can pencil through game.
Rotational or centrifugal forces and the effect it has on expansion of copper bullets is something I feel has not been explored and "investigated" fully by the shooting community.
For example a 150gr. 30Cal from a 12 or 14 twist at a velocity in the lower ranges and the same bullet from a much faster twist and same velocity shot in ballistic gel should show clear difference in expansion if rotational energy has a significant contribution to bullet expansion.

For me this was never about finding a more explosive bullet....it is indeed what makes a bullet hit “hard”. So it could be done with any bullet...Solid copper would be awesome, but the alloy and construction of the bullet will determine if it is capable of coming apart at all vs. a softer lead projectile.

I think it’s a very well thought out addition and angle to the test, and I appreciate that! Thanks for the post. Any 6mm solid copper fragmenting bullets out there?

 

[David Milisock]

For me this was never about finding a more explosive bullet....it is indeed what makes a bullet hit “hard”. So it could be done with any bullet...Solid copper would be awesome, but the alloy and construction of the bullet will determine if it is capable of coming apart at all vs. a softer lead projectile.

I think it’s a very well thought out addition and angle to the test, and I appreciate that! Thanks for the post. Any 6mm solid copper fragmenting bullets out there?

Try Barnes.

 

[ckaberna]

I know Alex said he would never do a 13.5 twist barrel again, but I have a 6BR 14T I am shooting 55 grain Sierra BK’s in at 3820 FPS and i don’t think the SF (splat factor) could get any better. The above crow was 100 yards and had a U shaped circle of “parts” that was measured at over 20’ I can’t recall the exact measurement. No pics but I recently shot a rat, not mouse but rat, with it. My fiancé was watching, she actually spotted it, when I pulled the trigger it disappeared. We went to look and found the hole in the dirt where it was sitting, she was laughing saying you missed it. All we ever found was a quarter sized chunk of “rump” with the tail still attached. So no, I didn’t miss I told her.......

 

[CharlieNC]

When the material properties are factored in the complexity goes up exponentially. Distributed forces along a rotating bullet of a given construction is where the material scientists come in and I lose my steam. I’m getting too old for differential equations!

I did calculate the total energy for both forward motion and rotation using a 55g 6mm bullet

Forward: 1490J @ 914.4m/s (3000fps)
2648J @ 1219m/s (4000fps)

Rotational: 4.19J @ 150,000RPM
16.75J @ 300,000RPM

So you are 100% correct that the forward kinetic energy dominates the rotational....but the rotational energy will send fragments out radially with 4x the force (when RPM doubles) once impact occurs. So the Fragment dispersion cone should be greater and dump more of that forward kinetic energy over a larger area.

Had to dust off the calculator! Feel free to check my math, the conversions can always bite you.

I think you are correct. Disregarding any fragmentation the rotational energy has to go somewhere, it doesn't simply disappear.

 

[AllThingsAI]

So the test planning and prep has started. Below is what I plan to do first (because it’s all stuff I have currently). It’s not ideal, but should give a decent first order test, and bullet construction of VMax bullets should be similar for both tests.

Goal is to track bullet fragment expansion pattern for each. If Fur alone doesn’t initiate expansion I will add a thin layer of modeling clay to the fur.

Test 1:
.224 Caliber 55g VMax Flat Base
Velocity ~ 3100fps (Muzzle)
100 yards
Variables: 1:12 twist (186K RPM) and 1:8 twist (279K RPM)
Rifles used (.223AI and .223)

Test 2:
.243 Caliber 58g VMax Flat Base (I don’t have 55s)
Velocity ~ 3100fps (Muzzle)
100 yards
Variables: 1:12 twist (186K RPM) and 1:7.7 twist (290K RPM)
Rifles used (6BR and 6mm Creedmoor)

Test Media:
6”x6” slotted rack
- Rabbit Fur (to initiate expansion)
- 1/4” air gap
- construction paper
-1/4” air gap
- repeat to 4”

Attached is a pic of my slotted media rack with slots every 1/4”


Stay tuned.

 

[AllThingsAI]

I think you are correct. Disregarding any fragmentation the rotational energy has to go somewhere, it doesn't simply disappear.

And just to give you a visual for what 16J of energy is. Take a 3.5 pound weight, raise it up 39 inches (1M) and drop it. It will hit the ground with 16J of energy.

 

[Tommie]

-watching

@Forum Boss this might be a great article

 

[PigButtons]

I'm watching too. Can't wait for some results.
By the way AllThings, is the AI for Artificial Intelligence or Ackley Improved, or something else?

 

[AllThingsAI]

I'm watching too. Can't wait for some results.
By the way AllThings, is the AI for Artificial Intelligence or Ackley Improved, or something else?

All Things Ackley Improved.....I’m a fan of the AI methodology, and I hate to trim cases! But I also am An Electrical Engineer by training and work with Artificial Intelligence and Machine Learning uProcessors some in my day job...so It was a good double meaning.

 

[AllThingsAI]

Target set-up:

Bullet Expansion Layer:
- Rabbit Fur, backed with 1/16” thick modeling clay, attached to card stock
Detection Layers:
- 16 layers of 64lb card stock spaced 1/4

Tomorrow I shoot, 2 days of prep for 4 shots! . Hopefully I can hit it!

 

[AllThingsAI]

Test 1 Summary: No expansion from either .224 55g VMax bullet. Bullets were both fired at ~ 3040fps from 1:12 and 1:8 twist 223’s. Clean holes all the way through both sets of test paper. I need to revisit the first layer of the target to get expansion initiated....clearly the 55g Vmax is tougher than anticipated.

Current first layer used rabbit fur, backed with 1/16” of modeling clay, then attached to card stock.....this yielded zero expansion.

Any thoughts on a better first layer expansion media? I’m going to scrap the fur, it’s a PITA and gets everywhere. I can just use bare leather instead.

I’m glad it was only 110 degrees outside today! I think I’ll wait for it to cool off a bit before I retest.