Twist rate vs killing power.

[Geo_Erudite]

Can you explain how you derived those figures for me?

Sure:

Bullet Mass:

M = w/g = (300 gr)/[7000 gr/lb * 32.2 ft/sec^2] = 0.00133 lb sec2/ft

32.2 ft/sec^2 = gravity

Kinetic Energy:

.5 * mass * angular velocity ^ 2

Angular Velocity:

2 * pi * bullet radius * revolution per second

[JackinSD]

Sure:

Bullet Mass:

M = w/g = (300 gr)/[7000 gr/lb * 32.2 ft/sec^2] = 0.00133 lb sec2/ft

32.2 ft/sec^2 = gravity

Kinetic Energy:

.5 * mass * angular velocity ^ 2

Angular Velocity:

2 * pi * bullet radius * revolution per second

Now that you have made the edit of your original post, I don't need to know how you came up with the numbers. They were originally not done correctly is why I was asking. They were 25-35 times what they should have been. Thanks for correcting them.

[Geo_Erudite]

Now that you have made the edit of your original post, I don't need to know how you came up with the numbers. They were originally not done correctly is why I was asking. They were 25-35 times what they should have been. Thanks for correcting them.

Yeah, I forgot to include our dear friend gravity in the kinetic energy equation, sorry about that.

[JackinSD]

Yeah, I forgot to include our dear friend gravity in the kinetic energy equation, sorry about that.

I saw the numbers and knew they had to be way out of whack. Did the calculations myself and was wondering if I had done something wrong. It had been a long time since I had used some of that math.
ha.

[davemuzz]

Numbers is what numbers is.......but those numbers don't pass the "smell test." If barrel twist had that big of an effect on down range energy, I would think I would have read a whole bunch about the subject in all of the books, articles, and gun rags I've read in the last ten years.

There may be a difference on paper, but the real difference in the field is too small to measure.

dave

[jb6.5]

I watched a dvd that barnes gives out. From what they claim, and show on a super slow motion film the bullets rotation does in fact cause trauma. Seems like the more spin the more it would tare up. The ballistic jell showed swirls spiraling off the bullet from the rotation through the whole block. Pretty cool to watch.

[Geo_Erudite]

Numbers is what numbers is.......but those numbers don't pass the "smell test." If barrel twist had that big of an effect on down range energy, I would think I would have read a whole bunch about the subject in all of the books, articles, and gun rags I've read in the last ten years.

There may be a difference on paper, but the real difference in the field is too small to measure.

Fair enough, I understand your point of view.

[JackinSD]

I watched a dvd that barnes gives out. From what they claim, and show on a super slow motion film the bullets rotation does in fact cause trauma. Seems like the more spin the more it would tare up. The ballistic jell showed swirls spiraling off the bullet from the rotation through the whole block. Pretty cool to watch.

What you are seeing is a very small percentage of the actual energy released within the animal, in the 1-2% area. No matter how fast you twist the bullet out of a normal rifle barrel.

Yes, you get more energy with a faster twist. However, the rpm of the bullet (energy) within a given caliber at various twist rates have an almost nonexistent difference.

More energy? Yes. Enough to see a difference? No.

[Geo_Erudite]

Really this comes down to two points of view, which neither is incorrect. One side of the debate you have the reduced complexity crowd, which realizes it is the basic components that describe what is going on. Jack is probably correct that 98% of what is seen can be explained by linear velocity and energy at the point of impact that carries through as the bullet moves through the animal. The other side of the debate realizes that the other 2% that isn't explained is a form of deterministic chaos. This side of the debate sees that the 2% not explained can have huge ramifications that escalates the amount of "damage" done (e.g. a bullet with a faster twist rate and higher energy rips an artery in an animal that a slower twist rate and lower energy would not rip).

[JackinSD]

Really this comes down to two points of view, which neither is incorrect. One side of the debate you have the reduced complexity crowd, which realizes it is the basic components that describe what is going on. Jack is probably correct that 98% of what is seen can be explained by linear velocity and energy at the point of impact that carries through as the bullet moves through the animal. The other side of the debate realizes that the other 2% that isn't explained is a form of deterministic chaos. This side of the debate sees that the 2% not explained can have huge ramifications that escalates the amount of "damage" done (e.g. a bullet with a faster twist rate and higher energy rips an artery in an animal that a slower twist rate and lower energy would not rip).

You could well be right on that also.

[TC260]

Kinetic Energy:

.5 * mass * angular velocity ^ 2

Where did you get this equation from? It doesn't make sense. The resulting units are Kg/s^2 which doesn't equate to anything. The equation for rotational energy I'm familiar with is .5*moment of inertia*angular velocity^2. Even still I don't think rotational energy is useful here. Centrifugal force I think is the property we should be looking at. In some cases it's obviously a strong enough force to rip the bullet apart in mid flight.


I watched a dvd that barnes gives out. From what they claim, and show on a super slow motion film the bullets rotation does in fact cause trauma. Seems like the more spin the more it would tare up. The ballistic jell showed swirls spiraling off the bullet from the rotation through the whole block. Pretty cool to watch.

Is that the same video where they talk about the bullet "augering" it's way through the medium?

[Geo_Erudite]

Where did you get this equation from? It doesn't make sense. The resulting units are Kg/s^2 which doesn't equate to anything. The equation for rotational energy I'm familiar with is .5*moment of inertia*angular velocity^2. Even still I don't think rotational energy is useful here. Centrifugal force I think is the property we should be looking at. In some cases it's obviously a strong enough force to rip the bullet apart in mid flight.

Here (http://www.jbmballistics.com/ballistics/topics/energy.shtml), but you can use any unit you want in the equation as long as it is the same consistent unit.

I think that torque makes better sense, but I don't have the table in front of me to calculate those values.

[slugslinger]

I'll agree with squirrelsniper; of all that factors that affect wound channel, I suspect rpm at impact is down a few on the list (unless of course the bullet is actually unstable/tumbling at impact). FPS, energy, bullet weight, bullet type/shape/composition, and especially shot placement and animal bulk/density would be more critical. RPM is a factor, yes, but not the predominant one . . . but evaluating ALL the factors is what makes it fun, right!!

[TC260]

In the link provided, (1/2)mv^2 is the equation for linear kinetic energy + (1/2)Iw^2 which is the equation for rotational energy where I = moment of inertia, w = "omega" which is angular velocity. For "I" you'd use something like what's on this list here http://en.wikipedia.org/wiki/List_of_moments_of_inertia

[gerard488]

Someone please correct me if I1m wrong but here`s what I came up with on twist rate:
Any bullet leaving a 1:12 twist barrel at 2800 ft/sec is spinning at a rate of 2800 revolutions/second which equals 168000 RPM
Any bullet leaving a 1:8 twist barrel at 2800 ft/sec is spinning at a rate of 4200 revolutions/second which equals 252000RPM
Another way is to use the formula for bullet rpm which is MV x 720/twist rate in inches( 2800 x 720 /12 = 1680000 )
That is a difference of 84000 rpm at the muzzle. Enough to make a difference? Maybe

[jb6.5]

I think rpms have a good bit to do with the damage a bullet does. Several post mentioned ke, but I know some bullets aren't made to be pushed above a certain fps, if so they will come apart. Its not the fps its the rpms. I'm not an expert on ke and all the factors that goes along with it. I can say I've shot alot of critters and took notice to what bullet has done what. Something makes the creedmoor cause more damage than most guns we've used.

[big honkin jeep]

You guys can do all the math in the world there is a huge difference between bullet performance which can be calculated to a tee and putting down a game animal which is anything but absolute. Too many things to consider like the size of the animal and the construction of the animal. They all have different thickness of hide and bone and each organ has it's own characteristics as well. Throw in the different constitution (will to live and or flee) of each animal and you have something that numbers don't have an answer for. Often times a bone hit by even a comparatively small bullet of any type construction will turn into lots of high velocity bone shrapnel and the wound channel will look like it was done by a hand grenade.
I really think the fundamentals of proper caliber and shot placement are what gives you the best odds, and would use whatever twist stabilizes the bullet and provides the most accurate shot placement.
Just another .02

[TC260]

I really think the fundamentals of proper caliber and shot placement are what gives you the best odds,

Rereading the thread, I don't see anyone proposing anything contrary to that. It's just some speculation as to why a medium sized round (6.5 CM) seems to have outsized performance on game. A little basic physics just adds some information to the discussion. Now if we'd broken down the bullet shape and integrated it to actually calculate centrifugal force to a tee then I'd agree that "the math" had gone too far :p

[stangfish]

The earth does not spin faster at the equator than it does at the polls. Faster twist rates do more damage because the bullets act like saws after they open.:cool:

[jb6.5]

Yep