# Thread: bullet velocity vs. expansion physics

1. ## bullet velocity vs. expansion physics

Hey fellas, can someone here help me understand what's going on inside the bullet's head. We were talking about how a bullet at close range (high velocity) will (often) not expand. I understand the common sense end of it (bugs on the windshield), but what are the physics involved? Is there some type of "breaking point" at which the bullet will expand? Is there a certain velocity the bullet must be travelling when it meets a mass of certain density? Is there a formula to figure it out? The conversation that brought this on is below. Anyone have an intelligent explanation?

Originally Posted by Smitty of the North
BucknRut:
I wouldn't dispute that, but I doubt it is a function of close range. I'll go with your explanation.

"If there is nothing of substance to trigger the expansion of the bullet, it will not expand. The collision with hide and organs is not always enough action to cause a reaction with the bullet."

I remember shooting at a soda can with my 280 once. (at close range) I thought I was missing it, because it didn't even move. I was pleased to discover that I hit it every time.

Smitty of the North
I understand and believe what you are saying is true. We will have to get someone else to help us better understand the physics of it. There has got to be some way to figure out the "breaking point" at which a bullet will expand. Maybe the guys on the shooting forum can help us out.

2. there are a couple differnt techniques used to make a bullet expand, first the hollow point. the hollw point relies on hyduralic force to start its expansion. if there is no liqiud or coliodial fluid to begin its expansion there is a very strong chance that expansion will not take place regardless of velocity. if you were say shooting through a stick/branch/log the wood could plug the point and cause the bullet to not expand. shoot into a log and recover your bullet, you'll see what i mean.

the second example is lead pointed bullets, they rely on impact to begin expansion. the shape is disrupted by the impact and the bullet drives through itself to some extent causing expansion.

the rate of expansion typically goes up with velocity and penetration does down on a scale of diminshing returns, there is a window of efficiency that the bullet expands reliably and still gets fair penetration. I have never seen a bullet over driven to a point of no expansion. I have heard of bullets going too fast and breaking apart. I personally have shot big animals with a 257 wby chronoed at 3600+ fps using barnes tsx bullets and find one wound channel and my bullets must be holding togethre well, I can't say for sure as I have never recoverd one in that caliber. I have recoverd a 180 gr tsx at 3400fps from a bull hit at less then 100yds it was mushroomed perfectly. weight was 178 grs. I have recovered a 225 gr 338 bullet that was shot into a deer at less then 100 yards, it too was well expanded, a lead pointed bullet. the trip that bullet took beat it to death and it was only 130ish grains when recoverd.

there are varying windows of velocity from the mfr's some shoot very fast and others very slow, the bullets should be matched to the case for optimum results. I can't see putting a barnes bullet in a 270win and expecting something supernatural, as every bullet in that caliber was designed for the velocity it travels, not the case with the 270 wby. 308 and 7mm bullets operate on such a wide velocity range you can only imagine how critical bullet choice would be.

so for the answer to your original question, over driving a bullet typically hurts penetration, but not expansion, underdriving a bullet typically hurts expansion but not expansion. both of course have other variables involved.

3. highcountry:

Absolutely. And that just about covers it.

It just seems strange that you can hit an animal at close range where the velocity is the highest and blame no expansion on high velocity.

I agree, that it takes some resistance to cause expansion at any velocity. The "soda can" offered little resistance, but I would think hitting an animal anywhere would be enough to cause SOME expansion.

That's why I said I thunk it strange in my first post, and later suggested the, complete penetration with no expansion, must have been due to something besides velocity.
Smitty of the North

4. Originally Posted by highcountry
so for the answer to your original question, over driving a bullet typically hurts penetration, but not expansion, underdriving a bullet typically hurts expansion but not expansion. both of course have other variables involved.
expansion but not penetration?

5. Originally Posted by bgreen
expansion but not penetration?

exactly, good catch

6. ## Thanks highcountry

Originally Posted by highcountry
so for the answer to your original question, over driving a bullet typically hurts penetration, but not expansion, underdriving a bullet typically hurts expansion but not expansion. both of course have other variables involved.
Thanks for the explanation highcountry. Very thorough and clears up exactly what I was confused about. In regards to the above (I understand the expansion/penetration typo), I am going to have to put my "pass-through" experience in the "non-typically" catagory. I can honestly say that the exit hole was the same size as the enter hole (it was high in the cavity above any plumbing). I have heard of others saying this, but have only seen mine. The deer never blinked on the first shot, but the second...well, he didn't blink that time either and we ate well that night
Thanks, buck

7. ## Hey BucknRut

I sent you PM. It is not related to this post, but thought I would let you know it is there.

8. ## Bullet Velocity Vs Expansion Physics

Everybody seems to have the right idea. I thought I would bring up one more important point.

A bullet traveling at a higher rate of speed through the same medium(density and width) as a slower bullet has less time to expand. Less expansion, less tissue damage.

I don't know how to explain it, but if we compare the 06 with the 300 win mag. My real experience.

The 06 will kill a deer deader inside of 200 yards than the 300 win mag. will. but the 300 win mag will kill a deer deader from 300 to 400 yards as compared to the 30-06.

I think killing power has a great deal to do with the time for expansion(velocity) and the width of the animal you are shooting through.

Slower speeds produces more drag and more friction. More friction, more heat, more damage.

I have shot a few deer inside 100 yards with expanding bullets with a 300 win mag. little hole in, little hole out. Deer runs forever. Good luck.

Same size deer, same size/type bullet, same range with the 30-06 and the deer drops.

The bullet in the 30-06 has more time to transfer all its energy due to slower velocity and time in the deer than the 300 win mag does.

But as stated before, take a thicker animal like an elk or a moose and do the same test. I would choose the 300 win mag, for it will spend about the same amount of time in and elk or moose as the 06 did in the whitetail at 100 yards.

I think the military did plenty of testing back in the early 1900's. There must be something behind the size/diameter, weight and speed of the .30-06 caliber bullet for them to make it their weapon of choice.

Laymen's terms, I know,

Good Hunting

KatzMO

9. Originally Posted by KatzMO
Everybody seems to have the right idea. I thought I would bring up one more important point.

A bullet traveling at a higher rate of speed through the same medium(density and width) as a slower bullet has less time to expand. Less expansion, less tissue damage.

I don't know how to explain it, but if we compare the 06 with the 300 win mag. My real experience.

The 06 will kill a deer deader inside of 200 yards than the 300 win mag. will. but the 300 win mag will kill a deer deader from 300 to 400 yards as compared to the 30-06.

I think killing power has a great deal to do with the time for expansion(velocity) and the width of the animal you are shooting through.

Slower speeds produces more drag and more friction. More friction, more heat, more damage.

I have shot a few deer inside 100 yards with expanding bullets with a 300 win mag. little hole in, little hole out. Deer runs forever. Good luck.

Same size deer, same size/type bullet, same range with the 30-06 and the deer drops.

The bullet in the 30-06 has more time to transfer all its energy due to slower velocity and time in the deer than the 300 win mag does.

But as stated before, take a thicker animal like an elk or a moose and do the same test. I would choose the 300 win mag, for it will spend about the same amount of time in and elk or moose as the 06 did in the whitetail at 100 yards.

I think the military did plenty of testing back in the early 1900's. There must be something behind the size/diameter, weight and speed of the .30-06 caliber bullet for them to make it their weapon of choice.

Laymen's terms, I know,

Good Hunting

KatzMO
Not to dispute your experience in the hunting field, that performance is always subjective and up to the animal, but your concept of expansion and velocity is just about bassackwards, no disrespect meant.

A bullet of some structual integrity that is designed to expand (soft point), will expand more rapidly when impact velocity is higher, or when the media is tougher, all else considered equal. What usually happens when velocity is high, the range is close, and the bullet isn't up to the job, the bullet is destroyed on impact or shortly after and doesn't get to the vitals.

The opposite scenario is also quite common, impact velocity is too low to initiate expansion (for this particular bullet) and a small exit wound is noticed.

When you hunt a lot in African countries you get to see the effects of different bullets. The reason being, the trackers are so good they can follow even a slightly wounded animal to the ends of the earth and you will get to see the affects of the first shot. I've seen many of these when hunting with others who were using various bullets from various calibers, as well as my own hunting experience. It is without a doubt, in my mind, that impact velocity must be matched to the bullet construction and to the animal being hunted. There are some bullets that work well over a broad spectrum of both velocity and animals and others that work well in a rather narrow niche, a small range of impact velocities.

I have formed an opinion, naturally, based on viewing the results of well over a hundred animals so taken. I have however a limited experience with some bullets and rather extensive experience with others. This also includes many different calibers and therefore different impact velocities.

Some bullets perform well, very predictably, when kept with in their design limits and fail miserably when driven at velocities above their limit. There are, however, a lot of very good bullets out there today and many that will stand the higher impact velocity of the super magnums. Using a caliber appropriate for the animal hunted and making a good shot can make almost any bullet look good. On the other hand, when we "over work" a bullet by using an inadequate caliber and weight and drive it at the speed of light, relying on it's super energy levels to kill like the hammer of Thor, we are often rewarded with a wounded animal, even with good marksmanship.

Life, and in this case, death is a compromise, and every system has it's limits.

10. ## Since "physics" is in the thread title...

Originally Posted by KatzMO
Slower speeds produces more drag and more friction. More friction, more heat, more damage.
Actually, higher speeds produce greater friction. Just ask the guys who make heat tiles for the space shuttle...

And I'm fairly certain the lethal damage bullets do comes from kinetic energy - displacement of matter at speeds great enough to rupture cell walls and other tissue. There's certainly friction, but bullets don't kill by heat.

That said, I'm at a loss to explain the small holes in and out by high velocity bullets at close range. I don't shoot any really high velocity chamberings, so I haven't seen it myself.

11. ballistic coefficient (higher the bc the flatter the trajectory):

C = W / [i x d^2]

where C= ballistic coefficient, W= weight of bullet (mass?), i= form factor, and d= diameter of the bullet.

through deformation (expansion) the energy is transfered to the target.

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