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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons, particularly in larger calibres (12.7+mm).

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Energy vs. Momentum   General Military Discussion

Started 5-Aug by JesseH1234; 3286 views.
JesseH1234

From: JesseH1234

5-Aug

Working theory: "kinetic energy" is mostly meaningless in the context of fired projectiles.  With about two important exceptions. 

For a while I have been struggling to understand what the relationship is between energy and momentum with regard to firearms.  I know what they mean in the academic sense, but how that translates into the real world and the relationship between the two was always a bit foggy.  "Energy" just sounds cool and the numbers are big, but here's the thing: a 55 grain bullet with 1,200 ft-lbs of energy is definitely not going to move a 1,200 pound object one foot.  It's not going to budge it, in fact an AR15 will barely budge a bowling ball. 

For instance I was watching an Ian/Karl video where they shot an M-1 Carbine and a 5.45 Krink clone; the .30 carbine knocked plates and spinners with notably more force, even though the "energy" involved is pretty close.  It turns out, if you do the math, .30 carbine has more momentum than even full power 5.56.  It's even more stark with 9mm vs 5.7 for example; similar energy, but 5.7 will barely budge plates. 

Another fly in the energy ointment is that when it comes to dangerous game, everyone reverts back to the oldie but goodie big slow bullet calibers, when a lighter/faster bullet gun with similar energy would be a much handier and lighter kicking weapon.... which in itself presents another paradox.  All this "energy" cannot be free.  Maybe I am just dense but I doubt I was the only one a bit confused by this. 

Finally, it hit me: "energy" is a mathematical description of the force required to ACCELERATE a projectile to a particular velocity.  The squared function is a description of the fact that you need twice as much powder (or half the bullet weight) to gain a 50% bump in velocity, plus/minus any difference in barrel friction etc. 

The energy is also a description of the force required to decelerate the projectile, which sounds good, but with one important footnote: the air and material the bullet encounters is also resisting the bullet with exponentially more force the faster the bullet goes. 

Once a target/soil/building is encountered, this exponential energy probably does translate to extra friction between the bullet and medium it is passing through, but to my mind that is only important in two situations:

1.  You are designing a bullet to fail/fragment under the stress of impact, or

2.  You are designing an ultra high velocity round for armor penetration, ala a discarding sabot AP round, where the friction generated by the projectile energy is enough to create a melting contest between projectile and armor

The first instance is useful if one is designing a round for say law enforcement; 5.56 will often penetrate barriers and buildings to a lesser extent than 9mm.  But that is not necessarily good for a military round, where barrier penetration is a key component of its effectiveness (one of the most important IMO).  The best penetrating rounds are always the ones that are least deformed by the medium they are passing through; the "energy" that is absorbed by deforming the projectile is not only wasted but creates a less optimal shape for passing through stuff. 

This also explains why the light/fast concept is not necessarily good for weapons on the low end of the spectrum, ala handguns.  Real people and their clothing resist bullets far more than the ballistics gel tests would lead you to believe....gel is a comparison tool between rounds, not a representation of what it does in real life.  Anyway, there may be a reason why people haven't flocked to 5.7 the way one might think based on its paper stats, esp. with hotter loads ala Elite Ammo. 

The second instance above is important if you are going for an APDS type round.  Obviously. 

My conclusion: unless one is designing a round specifically for those first two instances, the projectile's momentum is a much more useful measure of the relative "power" of the round.   There is probably a momentum/sectional density ratio that would give a pretty good estimate of a non-deforming projectile's ability to go through stuff. 

I know this is going to generate a bajillion "but stopping power" type replies.  Trajectory flatness is also another consideration, but is a different conversation.   I'm really curious if someone can point out where my thinking is technically flawed; I'm trying to learn here. 

PS: been reading this forum for years; I posted a few times a while back under another name, but forgot/lost my login info so just made a new one. 

gatnerd

From: gatnerd

5-Aug

JesseH1234 said:

My conclusion: unless one is designing a round specifically for those first two instances, the projectile's momentum is a much more useful measure of the relative "power" of the round.   There is probably a momentum/sectional density ratio that would give a pretty good estimate of a non-deforming projectile's ability to go through stuff.  I know this is going to generate a bajillion "but stopping power" type replies.  Trajectory flatness is also another consideration, but is a different conversation.   I'm really curious if someone can point out where my thinking is technically flawed; I'm trying to learn here. 

Its essentially wrong in reality. 

Using Power Factor - which is a simple measure of momentum, we get the following.

.45 ACP 230gr @ 830fps = PF 190

10mm 200gr @ 1200fps = PF 240

5.56 62gr @ 2950fps (M4 velocity) = PF 182

Yet, despite having lower momentum, the 5.56 produces vastly nastier wounds and greater tissue destruction. It also has superior penetration of intermediate barriers. 

stancrist

From: stancrist

5-Aug

JesseH1234 said:

when it comes to dangerous game, everyone reverts back to the oldie but goodie big slow bullet calibers

When it comes to the most dangerous game, almost everyone sticks with small fast calibers.

Senators Question Army Chief on Supposed Polymer Magazine Ban ...

Red7272

From: Red7272

6-Aug

JesseH1234 said:

My conclusion: unless one is designing a round specifically for those first two instances, the projectile's momentum is a much more useful measure of the relative "power" of the round.   There is probably a momentum/sectional density ratio that would give a pretty good estimate of a non-deforming projectile's ability to go through stuff. 

Short answer is no, the long answer is bullet construction. 

Shoot a 45 ACP FMJ at someone and it's going to make a straight neat hole with straight through them. Unless it hits something important the immediate effect is often quite limited.  

Shoot a 45 ACP load that is basically an otherwise empty jacket containing 19 needle bearings at someone and you get 19 separate wound tracks and 19 chances of hitting something important.  It's not going to do a damn thing to a metal plate but a soft and squishy human is another thing entirely.

Back when I was shooting pigs ( 60 to 150 kgs and usually covered in a few cm of mud) the 130 grain speer varmint bullet disemboweled them. And to be absolutely clear, 95% of the time they dropped like a stone, the other 5% of the time it just made them mad.  There was also the chance of hitting a lot of mud and the wound being superficial. 

The 168 grain matchking on the other hand plowed straight on through and out the other side. If that bullet hit a shoulder bone the pig landed on its face and died soon after. It didn't expand and didn't really tumble either, just crunched on though bone, bowles and muscle and exited, even if it was lengthways through a 100 kg pig.

Humans on the other hand are usually less than a 40 cm wide with all the important bits in the back.  You can try to hit the important bits with a slow moving round nose, but a bullet that yaws or fragments is going to do a lot more damage to soft tissue and can also hit those important bits. 

Now back to your bullet hitting a bowling ball. A better example is a bullet trap grenade. That  nearly 1000 mps, 3.5 gram bullet is slowed down to about 60 mps within a few cm and nearly all that energy is transferred to the grenade that the bullet is now a part of. Some becomes heat or vibration, or even light, but most of it is transferred to pushing along the grenade. 

Mount a bullet trap on a gong and how far it moves will be a lot closer. 

gatnerd

From: gatnerd

6-Aug

JesseH1234 said:

Another fly in the energy ointment is that when it comes to dangerous game, everyone reverts back to the oldie but goodie big slow bullet calibers, when a lighter/faster bullet gun with similar energy would be a much handier and lighter kicking weapon.... which in itself presents another paradox.  All this "energy" cannot be free.  Maybe I am just dense but I doubt I was the only one a bit confused by this. 

Just to ruin this a bit more, but all of the 'dangerous game' cartridges also have a shit ton of energy and plenty of velocity. 

The most common, globally rated (from Africa to Alaska) dangerous game round is the .375 H&H.

300gr @ 2560fps = 4,365 ft/lbs of energy. 

QuintusO

From: QuintusO

6-Aug

If that were the case, I should be dead.

https://youtu.be/X7EWKiXct48

No, momentum does not equal "power". Not at all. Energy is a much better indicator, but it's not the only factor.

In reply toRe: msg 1
JesseH1234

From: JesseH1234

6-Aug

JesseH1234 said:

I know this is going to generate a bajillion "but stopping power" type replies.

It's almost like I'm psychic or something. 

QuintusO

From: QuintusO

6-Aug

Having a bunch of experts correct you doesn't mean you're psychic, it means you're wrong.

You think NOBODY here has considered momentum before? I've empirically tested it. There is no correlation to lethality. The theory doesn't even make sense because momentum is conserved.

You're not a poor persecuted genius riling up the academy, you're spouting factually inaccurate half baked thrice-debunked BS and wondering why there's pushback.

QuintusO

From: QuintusO

6-Aug

There's a very large disconnect in your logic where you ally the Newtonian elastic collision dynamics with lethality. They are not allied. The momentum of a .30-06 gunshot that stops in the target will only accelerate a 200lb adult male to a speed of about 0.2 miles per hour, yet that round is highly lethal. It is lethal because of the work it does on the target, and an object's capacity to perform work is expressed by its kinetic energy:

https://www.thefirearmblog.com/blog/2016/07/16/ballistics-201-introducing-new-way-thinking-terminal-effectiveness-force-energy-work/

https://www.thefirearmblog.com/blog/2016/07/17/ballistics-201-introducing-new-way-thinking-terminal-effectiveness-energy-budget/

Also, you make a very serious error in that you mistakenly take "ft-lbs" to mean the ability to move X pounds mass 1 foot. That is incorrect, "foot-pounds" is more properly called "foot-pounds FORCE", as it is an expression of the ability to do work equivalent to to that many pounds force exerted over one foot of distance.

RovingPedant

From: RovingPedant

6-Aug

Energy, momentum and power are all well defined and understood properties. I don’t know why you would feel the need to redefine them and put them in quotes.

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