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

[AllThingsAI]

Caution, a bit of simple physics here and it’s a bit nerdy. Proceed with caution!

I’ve been thinking a lot about what causes a bullet to “hit” hard. I realize this is a subjective term but it should be somewhat quantifiable. Straight forward physics mixed with some materials science and a bit of voodoo . I’m an engineer so I pretend to understand the forces/energy involved, both Centripetal (Centrifugal), and linear (forward motion). Both functions square the velocity terms (RPMs and ft/s)...so you double the speed (rpm or ft/s), the energy goes up by 4x....and so on.

Thought Experiment (channeling my inner Einstein):

Say you are firing a solid lead projectile which doesn’t require spin. If this projectile were to hit a Prairie Dog at 2000fps, what would happen? What if you hit it at 4000fps? Probably not a major difference, but there will be some difference given 4x greater energy. Bullet will mushroom or break apart and all pieces will continue forward. No way all this energy can get dumped in 2 inches of Prairie Dog, so a lot of the “hard hitting” affect is lost. Bullet fragments won’t spray out radially.

Now take that same projectile and fire it at 2000fps and spin it at 100,000 RPM. There is now a HUGE Centrifugal force trying to tear the projectile apart. As soon as it hits an object and begins to mushroom it flys apart from the spin because it lost its mechanical properties which held it together. The bullet spreads apart in a radial direction as well as a forward direction.

Increase the RPM to 200,000 and the force trying to tear apart the projectile is now 4x what it was in the previous example...that’s HUGE x 4...aka one angry hair triggered bullet! It just wants to come apart as soon as someone looks at it wrong. Touch the first hair on a Prairie Dog and boom it explodes instantaneously.

That all makes pretty good sense in my mind, but my question is how does forward velocity come into play in the angry bullet scenario? If I double the forward velocity to 4000fps, forward energy goes up by 4x...does this cause more explosion of a projectile? Probably some but to what degree? I want to figure this out with some experiments, it should be fun. Which matters more for a “hard hitting bullet”: RPM, Velocity Or both?

Help me design some experiments. I can chamber different twist barrels on my lathe.

Update 7/29/20: Test results start on 4th Page

 

[Intheshop]

Woohoo..... lead bullet varmint killing

Lead is about all I shoot,and at some pretty obscene velocity. Varmints are my favorite target.

How "sharp" any forward facing edges are on impact "seem" to be where the magic comes from. Most of my screamer loads are in slow twist barrels. Velocity is @ starting to medium jacketed speeds. To get these edges to maintain crispness it gonna take some playing around with the alloy. Ideally,you want the softest alloy that gets the job done so there's a balancing act going on.

You can't get terminal ballistics until the internal/external process is dialed in. It sorta IS black magic. Way too involved here. Even the size of the meplat isn't as important (within reason) as how crisp the edges are. Hitting a crow with a 55g 223 lead bullet running 3kfps..... both,crow and bullet are gonna come unglued.

Maybe not the data or vector you were asking about but..... lead,varmints,and velocity is what gets me up in the morning. Good luck with your project.

 

[AllThingsAI]

Woohoo..... lead bullet varmint killing

Lead is about all I shoot,and at some pretty obscene velocity. Varmints are my favorite target.

How "sharp" any forward facing edges are on impact "seem" to be where the magic comes from. Most of my screamer loads are in slow twist barrels. Velocity is @ starting to medium jacketed speeds. To get these edges to maintain crispness it gonna take some playing around with the alloy. Ideally,you want the softest alloy that gets the job done so there's a balancing act going on.

You can't get terminal ballistics until the internal/external process is dialed in. It sorta IS black magic. Way too involved here. Even the size of the meplat isn't as important (within reason) as how crisp the edges are. Hitting a crow with a 55g 223 lead bullet running 3kfps..... both,crow and bullet are gonna come unglued.

Maybe not the data or vector you were asking about but..... lead,varmints,and velocity is what gets me up in the morning. Good luck with your project.

Just used lead as an example to keep things simple...in practical testing I’ll use a Sierra Blitzking or VMax. Just as long as it’s the same for each test. But it would be interesting to see what a solid led projectile does...just don’t think I could get it to those velocities. Thanks for the reply!

 

[Intheshop]

Hey,you're the one who brought up lead(jk);

7-08. 140g RCBS with a few "enhancements. Very quickly done,the bases get put in a 5 gang fixture and milled to get the deadnuts square,and set length/weight.

Lube grooves,which don't have any lube in them (lube causes way more problems than it fixes) get "knurled" for lack of a better term,during roll sizing(straight line sizing causes more problems than it fixes).

Bore ride nose gets a bit of attention as well,which is extremely hard to put into words.... it has to do with the amt of jam you need on tuning.



P.S. I knew where your project was headed,just that lead.... be it wrapped in a condom or nekid,dosen't behave like most physics would lead you to believe.

 

[NC308]

So what you are saying is,

take two 308’s. Bothe shooting the same projectile. Same load. Same barrel length. But one has a 10 twist and one has a 12 twist. Then the ten twist would have more down range energy because it’s spinning faster?? Humm. Maybe. Your analogy may be better. I don’t think you would see a huge difference between a10 and 12 twist.

 

[Alex Wheeler]

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.

 

[AllThingsAI]

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.

Exactly, that’s my experience as well and the math supports it. But I want to quantify it so it can be optimized. My gut tells me that bullet length and cross-sectional area also come into play.

 

[AllThingsAI]

So what you are saying is,

take two 308’s. Bothe shooting the same projectile. Same load. Same barrel length. But one has a 10 twist and one has a 12 twist. Then the ten twist would have more down range energy because it’s spinning faster?? Humm. Maybe. Your analogy may be better. I don’t think you would see a huge difference between a10 and 12 twist.

Something like that. Probably start with two 6mm’s shooting the exact same bullet at the exact same speed using 1:8 and 1:14...just making sure the bullet is stable. Then shooting gel or something like that to observe the splash pattern and depth. Rinse and repeat with same twist different velocity.

 

[TC260]

22 caliber barrels are available in a wide range of twists . I shoot 52 gr pills out of a 14 twist 22-250. That would provide enough contrast to a 7.5t or faster, not sure what's the fastest available but regardless, a wide enough range to show results if it's going to.

 

[AllThingsAI]

22 caliber barrels are available in a wide range of twists . I shoot 52 gr pills out of a 14 twist 22-250. That would provide enough contrast to a 7.5t or faster, not sure what's the fastest available but regardless, a wide enough range to show results if it's going to.

I’m thinking I need to also explore bullet diameter and size so maybe use a 22cal with 55g bullets, and a 6mm with 55g bullets...same velocity different twist. Could also test short stubby bullets vs long sleek bullets. My gut tells me short and stubby will tend to explode more than the long sleek bullets. Just a hypothesis.
I need some help designing a target that will help visualize the affect easily.

Maybe:
— Layer of rabbit pelt
— 1/4” small air gap
— construction paper
— 1/4 small air gap
— construction paper
—continue layering.

I would love ideas and suggestions.

 

[David Milisock]

Caution, a bit of simple physics here and it’s a bit nerdy. Proceed with caution!

I’ve been thinking a lot about what causes a bullet to “hit” hard. I realize this is a subjective term but it should be somewhat quantifiable. Straight forward physics mixed with some materials science and a bit of voodoo . I’m an engineer so I pretend to understand the forces/energy involved, both Centripetal (Centrifugal), and linear (forward motion). Both functions square the velocity terms (RPMs and ft/s)...so you double the speed (rpm or ft/s), the energy goes up by 4x....and so on.

Thought Experiment (channeling my inner Einstein):

Say you are firing a solid lead projectile which doesn’t require spin. If this projectile were to hit a Prairie Dog at 2000fps, what would happen? What if you hit it at 4000fps? Probably not a major difference, but there will be some difference given 4x greater energy. Bullet will mushroom or break apart and all pieces will continue forward. No way all this energy can get dumped in 2 inches of Prairie Dog, so a lot of the “hard hitting” affect is lost. Bullet fragments won’t spray out radially.

Now take that same projectile and fire it at 2000fps and spin it at 100,000 RPM. There is now a HUGE Centrifugal force trying to tear the projectile apart. As soon as it hits an object and begins to mushroom it flys apart from the spin because it lost its mechanical properties which held it together. The bullet spreads apart in a radial direction as well as a forward direction.

Increase the RPM to 200,000 and the force trying to tear apart the projectile is now 4x what it was in the previous example...that’s HUGE x 4...aka one angry hair triggered bullet! It just wants to come apart as soon as someone looks at it wrong. Touch the first hair on a Prairie Dog and boom it explodes instantaneously.

That all makes pretty good sense in my mind, but my question is how does forward velocity come into play in the angry bullet scenario? If I double the forward velocity to 4000fps, forward energy goes up by 4x...does this cause more explosion of a projectile? Probably some but to what degree? I want to figure this out with some experiments, it should be fun. Which matters more for a “hard hitting bullet”: RPM, Velocity Or both?

Help me design some experiments. I can chamber different twist barrels on my lathe.

There is no rotation aspect to the energy formula.

There is no rotational aspect to ballistic coefficient formula. Ergo the same BC projectile (assuming minimal stability rotation has been achieved) would deliver the same energy on target regardless of twist.

To test your concept you would have to send the same projectile at the same velocity (on target) from different twist barrels measuring velocity at target and observe results in a control medium.

In my observation a projectile can only impart energy at the target by maintaining significant structural integrity until it collides with bone or soft tissue.

My 223 and 6mm at 3,900 FPS with VMAX projectiles produce significant primary and secondary wound channels due to the rapid expansion. With head shots at 300 yards on chucks the bullets severely fragment after striking the skull. Producing acshalliw primary wound channel and a significant secondary wound channel, with targets as small as a chucks head all thecway through with small bullet fragmants thrlughout. The same range shot on the body produces less violent expansion and significantly less secondary wound channel with zero bullet fragment recovery.

The 6mm with a hide hunting bullet at nearly the same velocity and the same twist produces nearly a zero secondary wound channel so the hide has minimal damage. The bullet pass clear through.

My other observation is shooting in a rain, velocities as mentioned above have blown up my projectiles in the rain before reaching 100 yards. Also light fast projectiles in highntwist barrels have simply blown up on their own in flight.

 

[AllThingsAI]

There is no rotation aspect to the energy formula.

There is no rotational aspect to ballistic coefficient formula. Ergo the same BC projectile (assuming minimal stability rotation has been achieved) would deliver the same energy on target regardless of twist.

To test your concept you would have to send the same projectile at the same velocity (on target) from different twist barrels measuring velocity at target and observe results in a control medium.

In my observation a projectile can only impart energy at the target by maintaining significant structural integrity until it collides with bone or soft tissue.

My 223 and 6mm at 3,900 FPS with VMAX projectiles produce significant primary and secondary wound channels due to the rapid expansion. With head shots at 300 yards on chucks the bullets severely fragment after striking the skull. Producing acshalliw primary wound channel and a significant secondary wound channel, with targets as small as a chucks head all thecway through with small bullet fragmants thrlughout. The same range shot on the body produces less violent expansion and significantly less secondary wound channel with zero bullet fragment recovery.

The 6mm with a hide hunting bullet at nearly the same velocity and the same twist produces nearly a zero secondary wound channel so the hide has minimal damage. The bullet pass clear through.

My other observation is shooting in a rain, velocities as mentioned above have blown up my projectiles in the rain before reaching 100 yards. Also light fast projectiles in highntwist barrels have simply blown up on their own in flight.

The rotational force is most certainly there, simple physics. It indeed takes energy to induce spin, so there is an energy component with rotation. Like you mentioned the rotational component is what causes a bullet to explode...even with a raindrop if if too much force is applied. I want to know what the thresholds are. Once a bullet hits fur if it explodes radially (CF force) more energy will be dumped in a shorter span vs. a projectile that stays together or breaks apart with no radial (or minimal) component.

But that is what testing is for. As for gel, I’m not sure if it will actually show the affect I’m trying to test. I’ve never used it, but have seen lots of videos and articles. Pros and cons on each side. I think most people use it because it’s easy.

As for the Fur saving bullet vs hunting bullet comment you are correct, but that’s not an apples to apples comparison. The bullets have different mechanical properties. You can also spin that hunting bullet ALOT faster than a varmint bullet without it coming apart, so you could test a lower RPM hunting bullet with a much higher RPM hunting bullet...you just have to compare the same type of bullets.

 

[AllThingsAI]

I’m going to coin a new term and call it “Smack Factor” or SF for short.

Maybe a high def audio analysis at the point of impact as well as the visual target analysis. Look (or listen for how much sound energy is being generated by the impact. That would be pretty awesome.

I’d love for all my Varmint rifles to have optimized “Smack Factor”! That just sounds cool.

 

[CharlieNC]

I think the over-riding factor is based on the bullet design, what is it's response. Nothing, expand, fragment, etc? That will greatly affect energy transfer, wound channel, pass through, or not.

 

[Sniper338]

Put math aside and my thought process tells me a faster spinning projectile will want to come apart faster than a slower spinning projectile, all your looking for now is a object to induce the faster spinning projectile to fragment and explode... this equals varmint grenades

 

[AllThingsAI]

Put math aside and my thought process tells me a faster spinning projectile will want to come apart faster than a slower spinning projectile, all your looking for now is a object to induce the faster spinning projectile to fragment and explode... this equals varmint grenades

Yep, I just want to quantify it, that’s all. Something fun to do, no other practical reason except Maybe to try and save barrel life. No need to push a bullet at 4000fps Low RPM when 3300 High RPM will give you the same “Smack Factor”!

 

[jepp2]

No need to push a bullet at 4000fps Low RPM when 3300 High RPM will give you the same “Smack Factor”!

I'm intrigued. Guess I have always considered velocity trumps rotation. I probably would have named the SF as "splat" factor.

In your testing, you will probably encounter there is a transition point where the explosive nature the bullet turns into just mushrooming. This is for both your velocity and rotation aspects, and those might occur at different % of max. The example that comes to mind is the illustration of Nosler Ballistic tip bullets.

 

[AllThingsAI]

I'm intrigued. Guess I have always considered velocity trumps rotation. I probably would have named the SF as "splat" factor.

In your testing, you will probably encounter there is a transition point where the explosive nature the bullet turns into just mushrooming. This is for both your velocity and rotation aspects, and those might occur at different % of max. The example that comes to mind is the illustration of Nosler Ballistic tip bullets.
View attachment 1191180

Yep I thought about “Splat Factor” but I went for the more manly and better rhyming “Smack Factor”! That name is probably the best thing to come out of my madness! . I’ve seen that from Nosler and it’s a good illustration...my hypothesis is higher twist rates cause this to happen lower in velocity and more radial dispersion of the fragments...hence more “smack” or “splat” factor. Just need to figure out a method of testing. I need the experts to chime in to help me design a good target.

 

[Tommie]

Anyone else read the chapter in PO Ackley’s book about shooting feral burros?

 

[AllThingsAI]

Really cool video from the Slow mo Guys about the power of CF forces...just think a bullet is spinning way faster then they are spinning this light disk. It also shows a fantastic demonstration of a standing wave.

Slow-Mo Guys CD Spinning Video: