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Old July 28, 2018, 07:17 AM   #1
simonrichter
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weight vs. speed in terms of flight and terminal ballistics

I still don't get it in every aspect: Imagine you have a fmj .45 acp and a fmj 5.7 x 28 with the same muzzle energy, how would they differ in terms of flight path (ok, that one I can imagine...), penetration, energy transfer in soft medium etc.?

I reckon fat+slow means more drop and thus less accurate range, but what about energy retention after say 100 yrds?

Same for penetration - while it's logical that a bullet which is sleek and fast will penetrate better... On the other hand, while kinetic energy can easily be transfered, but bullets drawing their energy proportionally more from weight than speed keep that factor while crossing through a medium since the weight will remain the same (for a fmj)....
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Old July 28, 2018, 07:24 AM   #2
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Simon noted:
...“On the other hand, while kinetic energy can easily be transfered, but bullets drawing their energy proportionally more from weight than speed keep that factor while crossing through a medium since the weight will remain the same (for a fmj)....”


This is an incorrect assumption.
Kinetic energy is a product of mass and speed squared.
Zero mass or speed = zero kinetic energy.
Consider an electron or photon with near zero mass but moving incredibly fast.
Put many together constructively and the energy level can be massive.
Try s out a race car can easily have that same amount of kinetic energy.

It’s the product not just the mass or just the speed.
A classic physics question is “how many photons does it take to equal the same kinetic energy as a fully loaded 18 wheeler moving down the interstate highway?

The answer is left as an exercise to the reader.
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Old July 28, 2018, 07:57 AM   #3
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I reckon fat+slow means more drop and thus less accurate range
This is also a bad assumption. Projectile design and other factors can effect accuracy more. Also a very heavy bullet travelling at the same speed as a smaller one may likely travel farther. If you don't believe take an 8 ounce rock and 1 ounce pebble and throw them at the same velocity and see which one goes farther.

Quote:
bullets drawing their energy proportionally more from weight than speed keep that factor while crossing through a medium since the weight will remain the same (for a fmj)
You are also assuming that a larger projectile will retain more mass. This is a bad assumption as well.
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Old July 28, 2018, 08:30 AM   #4
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Yes, a projectile that is moving slower will have more drop per distance traveled since gravity affects all projectiles in the same manner, regardless of mass. But you must also consider ballistic coefficient, which is a function of sectional density and shape of the projectile. Sectional density is bullet mass divided by cross sectional area. So all other things being equal, a projectile with greater mass will have a more favorable ballistic coefficient. Which is another way of saying that a heavy bullet may travel farther.

But in a manner akin to the fable of the turtle and the hare, a projectile with a more favorable ballistic coefficient (think more aerodynamic) that starts out slower may catch up to and surpass (in terms of velocity) a projectile with a less favorable ballistic coefficient that had greater muzzle velocity.

When it comes to terminal ballistic performance, it is what is happening when the projectile hits the target that matters, not what is happening when it leaves the muzzle.
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Old July 28, 2018, 09:09 AM   #5
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Important

The above post is important. It helps me get my head around the physics when I remember that air is thinner than water.....well duh.

My point is that the air is a medium that must be penetrated. No it's not water, but it helps to think of it in that way.
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Old July 28, 2018, 11:38 AM   #6
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In a vacuum, speed would contribute more because, as has been noted, the basic forula is mass times velocity squared. So, twice the mass equals twice the energy, whereas twice the velocity equals four times the energy.

But, again as noted, bullet shape has some effect. In air, for a given bullet weight, a spire-point rifle bullet (like a 55-grain .223) will maintain velocity more efficiently as it travels that a hollow point bullet with a squared-off base, or (perhaps worst case) a wadcutter bullet (a "flying ash can"). This is why all ballistics calculators include a factor for the ballistic coefficient.
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Old July 28, 2018, 12:30 PM   #7
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Originally Posted by Aguila Blanca View Post
...a 55-grain .223) will maintain velocity more efficiently as it travels that a hollow point bullet with a squared-off base, or (perhaps worst case) a wadcutter bullet (a "flying ash can"). This is why all ballistics calculators include a factor for the ballistic coefficient.
And unfortunately for easy algebra, the ballistic coefficient changes with speed of the projectile.
That is not as obvious with short range, lightweight projectiles like 55gr 5.56, But is very obvious on a 750gr .50 cal.
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Old July 28, 2018, 01:09 PM   #8
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This is a complex issue with no one short answer that could be posted here. Handgun bullet all have such poor aerodynamics and are usually used at ranges so close that it simply isn't a factor. Aerodynamics can have a huge effect on rifle bullets fired at ranges greater than 50-75 yards. And the greater the range, the more important.

Assuming the same construction Sectional Density (SD) is the best predictor of bullet penetration. It is a factor of the bullets weight and diameter, but does not consider shape or construction. Generally speaking heavier bullets within the same caliber will be longer and penetrate better. But when comparing bullets of different diameter things get interesting. A 124 gr 9mm bullet of the same construction will match or beat a 230 gr bullet fired from a 45. A 150 gr bullet fired from a 270 will match or beat a 180 gr bullet fired from a 30-06. It's not just weight.

Ballistic Coefficient (BC) is similar to, but not the same as SD. BC is the measure of a bullets aerodynamics and as a rule heavier bullets are more aerodynamic because they are longer. But this can lead to confusion. A round nose 180 gr 30 caliber bullet has a BC of .241 and a SD of .271. A 180 gr bullet with a modern high BC shape has exactly the same SD of .271, but a much higher BC of around .507.

If we fire the RN bullet from a 300 WM at 3000 fps, and the high BC bullet from a 30-06 at 2800 fps both bullets will penetrate exactly the same because the SD is the same. But the much more aerodynamic bullet fired from the 30-06 will pass the 300 WM in velocity at only 75 yards. And as range increases will only widen the gap.

And then there is bullet construction. Bullets are designed to work within a range of impact velocities. Since handgun bullets are slower they are designed to expand between about 700-1200 fps. Too fast and they break up and don't penetrate. Too slow and they don't expand. And every bullet is different. Brand "A" 125 gr bullets may work fine from a 38, but blow up at 357 mag speeds. Brand "B" might work great in 357 mag, but not expand in a 38.

The same is true with rifle bullets. Most work well between about 1800-2800 fps. But once again each manufacturer designs their bullets differently and it is up to the consumer to understand that and not ask a bullet to do something it wasn't designed to do. Some still expand down to 1600 fps and others will hold together at well over 3000 fps. But you won't find any bullet that works well at both ends of the spectrum. Harder bullets need more impact speeds, soft bullets work better at slow impact speeds.

Bullets almost never actually fail. Shooters often don't get the results they want because they chose the wrong bullet. Some need at least 2000 fps to expand, but guys shoot them in rifles that only produce 2500 fps at the muzzle. At 200 yards they are doing less than 2000 fps, fail to expand and the shooter blames the bullet. Or they take a bullet not designed to work above 2800 fps and shoot it at 3000 fps into a deer. The bullet explodes and fails and once again the shooter blames the bullet.
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Old July 28, 2018, 01:30 PM   #9
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I reckon fat+slow means more drop and thus less accurate range, but what about energy retention after say 100 yrds?
Fat and slow, thin and fast, light and heavy all these things play a part but you need to reckon them in the right way.

Drop is the result of gravity. Everything on Earth drops at the same speed (acceleration due to gravity) How much drop a bullet has is a result of a constant acceleration downward due to gravity (falling), and the amount of TIME before the (falling) bullet reaches the target.

The slower the bullet the more time it takes to get to the target, so the more gravity pulls it down.

Retaining energy involved lots of factors, and one of the big ones is called momentum. Its a function of inertia. Heavy tends to want to keep moving more than light, and this is also a key factor in penetration, as well.

Muzzle energy (ft/lbs) is a calculated number, and it is a commonly used standard for comparison, BUT it does not take into consideration other, important factors, which have real world effect.

Compared to rifles, even the most powerful handguns are pretty low energy weapons. Energy is important, but size does matter too.

Math is wonderful, and proves many things, but math also give the same energy value to a small fast bullet as it does to one that is twice as big around, and 8 times the weight, moving slower.

Fast and light vs slow and heavy have been the two main schools of thought since fast and light became possible. Both work, but they work differently, and many other factors are as, or even more important.
Many of the factors that are important in long range shooting don't matter as much at pistol ranges.

Ought to be clear as mud, by now, but if not, I can keep going...
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Old July 31, 2018, 12:54 PM   #10
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Get a hold of a copy of Bob Forker's book Ammo & Ballistics. it's a very useful book. It has a number of articles explaining what's being discussed here.

It has another interesting feature which is that it has charts attached to each caliber bullet it describes that provides you with the ballistic features. This enables you to study if your assumptions are correct or not and to see some of the factors that effect bullets and ammo.

Much more useful than the internet. Books that is.

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Old August 2, 2018, 03:05 PM   #11
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Thank y'all so far. Yet, we kinda got stuck in the realm of external ballistics (aoart fro sone side notes). Any entries for terminal ballistics? Someone mentioned the role of momentum for terminal ballistics - does that mean a heavier bullet is more likely to "push through", at least given it had the same diameter than a lighter, faster bullet of the same energy?
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Old August 2, 2018, 05:03 PM   #12
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Quote:
Originally Posted by simonricher
Thank y'all so far. Yet, we kinda got stuck in the realm of external ballistics (aoart fro sone side notes). Any entries for terminal ballistics? Someone mentioned the role of momentum for terminal ballistics - does that mean a heavier bullet is more likely to "push through", at least given it had the same diameter than a lighter, faster bullet of the same energy?
It still isn't that simple, and the preceding discussion explains why. When you say "of the same energy" I assume you mean "of the same muzzle energy." But the ballistic coefficient affects how well a bullet maintains its velocity as it flies downrange. Since energy is a function of mass times the square of the velocity, it follows that a bullet that maintains velocity better downrange (better ballistic coefficient) also retains energy better downrange.

Using ballistic coefficient, it is possible to calculate fairly closely the velocity of a bullet at various distances from the muzzle. When you know the velocity at distance (say 200 yards, or 500 yards), you can then calculate the remaining energy at 200 yards or 500 yards, and compare the downrange energy of two different cartridges.

Drop due to gravity doesn't enter into this. Whether or not a cartridge has a flat trajectory or drops off steeply doesn't matter for this discussion. What matters is how well a bullet maintains velocity over distance.
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Old August 2, 2018, 10:32 PM   #13
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Some parts of it are simple, some get complex. Velocity changes throughout the flight of the bullet and its passage through a target medium. Since velocity is squared when calculating energy, small changes in velocity, make larger changes in the calculated energy.

So, literally, velocity, and therefore energy are moving targets. One set of numbers at the muzzle, another at every point downrange until striking the target, and, if you had the data for velocity change during passage through the target, you'd see it change there, as well.

MASS (or weight, in earth gravity) does NOT CHANGE (until/unless the bullet breaks up). The greater the mass, the greater the inertia. And it is inertia that makes the bullet "keep going".

Lots of other things have an effect on penetration, such as what the bullet is made of, and how, and what the target is made of, degrees of resistance, surface area, and more. An animal or human body has multiple types of tissue, and so many different levels of resistance. lots of things matter, some more than others.

Don't get hung up on energy being the sole yardstick of performance. Other things matter as well. What energy numbers do is provide a standard for comparison of other wise quite different things.

You can load a 50gr bullet in a .22-250 and a 400gr bullet in a 45-70, and get exactly the same energy number.

ASSUMING neither expands (or explodes) which do you think is most likely to "keep going" after hitting something, the small bullet moving very, very fast, or a bullet 8 times the mass, moving more slowly...

Here's another way to look at it... ever close a REALLY heavy door, like a safe door?? thick, heavy steel...

Slap that door with your hand, hard as you can...
ow! How much did it move? just a little, if any..
Now, PUSH the door...see it swing shut???

Same amount of energy applied, but the inertial of the door makes the difference. same forces at work as bullet penetration, just in the opposite direction.

Talk to some steel silhouette shooters, have them explain how a light fast bullet doesn't knock over the 500 meter ram like a slower heavier bullet even though they have the same energy numbers (ft/lbs)

Momentum does matter.
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Old August 3, 2018, 02:04 AM   #14
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Thank y'all so far. Yet, we kinda got stuck in the realm of external ballistics (aoart fro sone side notes). Any entries for terminal ballistics? Someone mentioned the role of momentum for terminal ballistics - does that mean a heavier bullet is more likely to "push through", at least given it had the same diameter than a lighter, faster bullet of the same energy?
In the first post here you asked about the 5.7x28 vs. the 45 acp.

Below is a vid of the 5.7x28 impacting a block of 20% ordnance gelatin. This is a 40 gr. bullet traveling at 1628 fps at point of impact. The block of gelatin is 10" long. Note the movement of the block.

https://www.youtube.com/watch?v=4U40ArShS6M

That vid is immediately followed by another round being tested against light body armor. Watch both a few times.

Here is the source of those vids where you can see more and different bullet weights and types. In all of these look for the depth of penetration and the expansion of the bullet. Look also at what happens to the block of gel when it is hit. Scroll to the bottom of the page for more vids.

http://www.brassfetcher.com/Handguns.../5.7x28mm.html

Now go over to here...

http://www.brassfetcher.com/Handguns.../45%20ACP.html

Here is the 45acp. Again look at the various type bullets and weights and how they perform and look at what happens to the block when struck. The blocks are the same as in the previous 5.7x28 vids.

Here is a 230 gr. Federal Hydra Shok striking 20% ballistic gel at 833 fps.

https://www.youtube.com/watch?v=OwAbIdamK2A

Other rounds are below.

https://www.youtube.com/watch?v=nt43BScUdt0

http://www.brassfetcher.com/Handguns.../45%20ACP.html

I suggest you take an hour and study these some.

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Old August 25, 2018, 09:45 AM   #15
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The late Col.Cooper said it best, as he preferred a medium weight bullet with appropriate velocity. Also, he preferred to carry a J-frame revolver if not the 1911.

BUT the best advise he dwelled on is PLACEMENT!
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Old August 27, 2018, 12:17 AM   #16
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This question and problem is rather complex. However, there are some essential concepts to consider.

Do not compare apples and oranges. Rifles (or carbines) with a velocity of 2000 fps and over are not to be compared with handguns of less than 1400 fps.

One does not get 'something for nothing'. In any arm with a specific pressure limit, one increases muzzle velocity by decreasing bullet weight and vice versa. To increase both bullet weight and muzzle velocity, one must move up to to an arm with greater pressure limits.

Penetration varies. Penetration in a single surface (car door or wood panel, for instance) depends principally on simple velocity (at impact, not muzzle). Penetration in a homologous mass (game animal, water, the odd villain) depends principally on momentum and sectional density of the projectile. Other factors are involved such as bullet shape and material.

Trajectory - bullet drop at a specific range - is a function of ballistic coefficient and initial velocity. However, a bullet with less drop is not particularly more 'effective' on game than a bullet with greater drop at the same range.

The desired application is quite important in making such determinations and decisions.
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