Question about guns and momentum

[Kinthalis]

Over at the Mass Effect forums there was a discussion about realism in the game. Mass Effect is a sci-fi game for the XBOX and PC by Bioware FYI.

Anyway, the discussion centered around a particular type of round that the player can use in their weapon called a hammer. Essentially it had the power to knock your opponent down.

People said this was not possible and explained the reason why thusly:

1. Real guns don't really knock people down. This is a Hollywood thing, the reason being...

2. If the round had enough power to knock the target down it would have to knock the shooter down as well given Newton's second law.

I'm not sure about number 1 as I have heard people discuss the stopping power or certain rounds, implying that they could knock an opponent back.

Number two may be right, I don't know. Perhaps, somehow, only a fraction of the energy is transmitted to the weapon as the round if fired?

So basically I'm looking to know if indeed real guns can knock opponents down. And if so, can they be used by a shooter without himself being knocked down, and if so, why? And would the story be any different if instead of chemically propelled rounds the gun in question used magnets (a rail gun) instead?

[Derleth]

No, real guns don't knock people down for precisely the reason you were told. And you misunderstand stopping power: A round's stopping power is related to how much damage it does, not (entirely) to how much force it knocks into the target. No non-explosive round is going to blow the target back but a round that doesn't pass through cleanly is going to cause enough damage and pain to make the target stop. Hence hollow-point rounds that mushroom and make gaping exit wounds.

[puppygod]

Short answer is no, real guns don't knock people down. Action equals reaction and all that.

Long(er) answer: it's more complicated than that. Stopping power per se is losely defined and unclear concept that is much overused to the level of being meaningless. Effectiveness of firearms depends mostly on perforation of vital organs and - in longer timescale - bleeding. On the other hand, there are weapons designed to incapacitate through blunt trauma. Rubber bullets and so on can effectively knock person down, although effect mostly depends on location of the hit and concentration of energy on relatively small spot. They still have heck of recoil, but when you know it and properly hold gun, it's not a problem to compensate for it.

But.

In the SciFi setting (and I understand Mass Effect is such setting) it's quite possible to imagine bullet that activates small rocket thruster upon contact with target effectively knocking it off without expending huge ammount of recoil on the shooter.

[beowulff]

Depending on the gun, I think it would be possible for a person to get knocked down when shot. Not likely from a handgun, but possibly from a rifle or shotgun.
When firing, the shooter is generally braced for recoil. The target is probably unprepared, and may be off balance. I've seen shooters get almost knocked over when firing a shotgun, when they were unfamiliar with the gun.

[Derleth]

I should add that stopping power is partially psychological: No shot that does not sever the spinal cord or damage the brain* is going physically force a target to drop instantly. The reason people do is because they know they've been shot and they decide that dropping instantly is the best option. Or, you know, they don't: In Stiff, Mary Roach tells of Captain La Garde, who wrote about the Moro tribesmen the American Army fought in the Phillippines back when they were an American colony. She states that he wrote of one such tribesman who advanced 95 yards while being shot by a whole guard.

*(Even damaging the brain isn't foolproof. A rail worker named Phineas Gage (http://www.deakin.edu.au/hmnbs/psychology/gagepage/Pgstory.php) who got an iron spike shoved into his forebrain survived for over a decade after, albeit with a much different personality.)

Deer are the same way. Animals don't know what it is to be shot, so they can travel for miles slowly bleeding from a mortal wound.

[Finagle]

And would the story be any different if instead of chemically propelled rounds the gun in question used magnets (a rail gun) instead?

Mebbe. If you accelerate the round over a longer duration, the total force will remain the same, but the impulse will be reduced. So firing the hammer wouldn't necessarily knock the shooter on his can. As a previous poster noted, a rocket propelled projectile would accomplish the same thing. Come to think of it, a rocket-propelled projectile where the rocket doesn't fire until it embeds itself in the target could do some serious knockback.

[Tripler]

To sing in the chorus: the kinetic energy of a projectile is focused on such a small point, that no, the bullet in and of itself will not knock you down but will (theoretically) pass through the body.

The energy imparted through the collision of the bullet to the body will generate force to help knock you down, but as said before, the bullet is meant to tear/shred/poke/destroy tissue in order to make the target stop.

Now, I can see the arguement where a bullet strikes a bone enough to impart such energy as to knock a target off balance or maybe even knock the individual around as to make him fall, but not stop the target cold unless it's a pretty heavy caliber (like a shotgun or large caliber rifle) spread out over the whole body.

Sorry, Hollywood doesn't write physical laws, (or gun laws for that matter).

Tripler
I tried to get Hollywood to repeal the law of gravity once--their Congress wouldn't even convene writer's strike.

[Paul in Qatar]

[QUOTE=Finagle]Mebbe. If you accelerate the round over a longer duration, the total force will remain the same, but the impulse will be reduced. /QUOTE]

The only way to sustain acceleration after firing is my using something like a rocket. This is why a rocket launcher can fire honking huge warheads. In any conventional gun all the energy a round is going to get comes in the first shakes of a lamb's tail. (An artillery round feels several hundred times the force of gravity applied at launch.)

So while a reduced recoil hammer is possible by a number of tricks, no conventional gun can achieve them as a practical matter.

As for being impractical, make the gun out of a very heavy metal. If you want to lug the beast around, you will hardly feel the recoil as it will have a hard time moving several hundred pounds of gun, let alone you.

[VunderBob]

I should add that stopping power is partially psychological: No shot that does not sever the spinal cord or damage the brain* is going physically force a target to drop instantly. The reason people do is because they know they've been shot and they decide that dropping instantly is the best option. Or, you know, they don't: In Stiff, Mary Roach tells of Captain La Garde, who wrote about the Moro tribesmen the American Army fought in the Phillippines back when they were an American colony. She states that he wrote of one such tribesman who advanced 95 yards while being shot by a whole guard.

You're ignoring the effects of hydrostatic shock, which can drop a target without significant direct trauma to the brain or spine. Won't happen every time, but is more probable with larger calibers and rounds that expand on impact.

[Peter Morris]

To sing in the chorus: the kinetic energy of a projectile is focused on such a small point, that no, the bullet in and of itself will not knock you down but will (theoretically) pass through the body.

And you might not even notice this until you look down and see a bright spot in the middle of your shadow, where the sunlight comes through the hole. :rolleyes:

[Terminus Est]

You're ignoring the effects of hydrostatic shock, which can drop a target without significant direct trauma to the brain or spine. Won't happen every time, but is more probable with larger calibers and rounds that expand on impact.
Which is why the M1911 (http://en.wikipedia.org/wiki/M1911) was developed. Its higher caliber proved useful in stopping said Moro tribesmen.

[Chronos]

Mebbe. If you accelerate the round over a longer duration, the total force will remain the same, but the impulse will be reduced.Other way around. If you have a really long barrel, you'll get the same impulse, but with a decreased force. But it doesn't matter what technology is accelerating the projectile down that barrel, just how long it is.

[Eben]

As an example of a weapon that would knock a person back if they were unlucky enough to be hit by it... the AT-4 (http://en.wikipedia.org/wiki/AT4) is a recoilless rifles that would most certainly knock a human back. The lack of any significant recoil is mainly due to the thrust from firing simply shooting out the back of the tube. This means you don't want any buddies behind you when you fire or you'll fry them.

As an interchangeable round in a weapon (as in Mass Effect, love the game!) that's not really an option.

[puppygod]

You're ignoring the effects of hydrostatic shock, which can drop a target without significant direct trauma to the brain or spine. Won't happen every time, but is more probable with larger calibers and rounds that expand on impact.

If you want to bring hydrostatic shock into discussion, better present a really solid cite. Evidence of existence of such phenomenon (at least for handgun bullet velocities) is very weak.

[Stranger On A Train]

Depending on the gun, I think it would be possible for a person to get knocked down when shot. Not likely from a handgun, but possibly from a rifle or shotgun.
When firing, the shooter is generally braced for recoil. The target is probably unprepared, and may be off balance. I've seen shooters get almost knocked over when firing a shotgun, when they were unfamiliar with the gun.I've fired into large wood stumps (~150 lbs) with pistol and rifle; even with a heavy rifle round or shotgun slug it'll only shift a few inches, and if fired well above the center of mass may make it slowly tip over. Shot or pistol rounds will just nudge such an inert mass.

For comparison, let's look at the momentum of different projectiles:
baseball: 10.3 slug*in/sec (5 oz @ 60 mph)
.357 Magnum: 10.2 slug*in/sec (180 gr @ 1060 fps)
.45 ACP: 11.0 slug*in/sec (230 gr @ 900 fps)
.308 Rem: 21.5 slug*in/sec (147 gr @ 2750 fps)
12 ga 3" Magnum slug: 41.0 slug*in/sec (1 oz @ 1760 fps)
.50 BMG: ~120 slug*in/sec (800 gr @ 3000 fps)

All of these weapons except for the last can be fired by a small-framed adult in a stable upright firing stance. Shooters who are knocked back by recoil are either not in a stable stance or (more frequently) have not been taught to properly hold the weapon. (Admittedly the 3" Magnum slug will beat you up pretty well after a few rounds.) The .50 BMG is typically fired from a bipod or vehicle swivel mount, and uses a large muzzle brake to reduce felt recoil; a normal person probably could not fire it from a standing position.

A 12 ga 3" Magnum slug might knock someone over, and a .50 BMG would probably lift someone up and toss them a bit (and also reduce a person to little more than a mass of hamburger), but any round capable of being fired from a light (infantry) rifle just doesn't have the momentum to lift or push a person into an otherwise steady target into unstable position. However, hit someone in the head or leg and there is a good chance that they'll fall over from shock or incapacitation.

"Stopping power" is a meaningless term from a technical perspective; forensic ballisticians tend to use the term "stopping potential" as a more valuable empirical measure of the effectiveness of various rounds.

You're ignoring the effects of hydrostatic shock, which can drop a target without significant direct trauma to the brain or spine. Won't happen every time, but is more probable with larger calibers and rounds that expand on impact.Hydrostatic shock resulting in persistent trauma doesn't occur in rounds moving less than ~1500 fps. Hydrostatic shock can be an issue with centerfire rifle, but not standard pistol trauma.

Stranger

[RaftPeople]

Is there some way we can work a treadmill into this question?

[trupa]

This is second hand, so take it with a grain of salt, mind you, but hunters often report that a shot dear will collapse immediately, and then "wake up" and run like the dickens after a few minutes. This phenomenon is theorized to be caused by mechanical shock temporarily compressing the upper spinal cord or brain stem when the bullet trajectory is close enough to either structure. It's believe to be an effect like a concussion or a knocked-out boxer. If this happens in a person, they would collapse, and maybe even fall backwards, thus giving the appearance of being knocked back, although they still wouldn't go flying back 3 feet.

[Crafter_Man]

The .50 BMG is typically fired from a bipod or vehicle swivel mount, and uses a large muzzle brake to reduce felt recoil; a normal person probably could not fire it from a standing position...
Incorrect.

I have a .50 BMG rifle. Serbu BFG-50. I shoot it in our backyard. (Seriously.) Just last Saturday I was shooting it in the offhand (standing) position, just for fun. It didn't lift me or throw me back. I just stood there and shot it like a normal rifle (albeit a lot heavier!). It was like shooting a 12 gage shotgun. And I'm not big... I'm only 5' 6".

In case you don't believe me, would you like me to videotape it and post it on youtube?

[Sam Stone]

Another factor you have to remember is that the gun itself has inertia and this will flatten the impulse curve. A .50 cal rifle is a heavy sucker, and it doesn't want to move much when hit with a sharp impulse, so it will act as a shock aborber.

A .45 colt pistol has a reasonable recoil that even small-framed people can handle. But take that .45 round and put it in a small derringer with not much mass, and it can break your wrist.

But inertia is also why bullets don't send people flying. The body resists being moved, so the energy of the bullet is expended by deforming itself, shattering bones, and creating hydrostatic shock waves. If it can't expend its energy doing that, it'll just punch right through the body. Even a .50 caliber bullet will do this before it will send a person flying. In fact, as I recall Mythbusters shot a man-sized dummy with a .50 cal, and the dummy was wearing armor plate to ensure that it absorbed all the energy of the bullet. even so, the dummy was only knocked back a few inches. You needed a high-speed camera to see the motion.

My gut feeling is that a projectile that will send a human flying would need to have mass which is a significant fraction of the weight of a person, traveling at reasonably low velocity (because if something that heavy was traveling at typical bullet velocities, it would just pulverize/vaporize the person). A 10 lb object traveling 100 mph, for example.

[matt]

The body resists being moved, so the energy of the bullet is expended by deforming itself, shattering bones, and creating hydrostatic shock waves. If it can't expend its energy doing that, it'll just punch right through the body

(SNIP)

My gut feeling is that a projectile that will send a human flying would need to have mass which is a significant fraction of the weight of a person, traveling at reasonably low velocity And there was another Mythbusters demonstrating that you can't even shoot someone's hat off with a pistol or rifle, for much the same reasons you've described!

[Crafter_Man]

A .50 cal rifle is a heavy sucker, and it doesn't want to move much when hit with a sharp impulse, so it will act as a shock aborber.Very true.

If it can't expend its energy doing that, it'll just punch right through the body. Even a .50 caliber bullet will do this before it will send a person flying. In fact, as I recall Mythbusters shot a man-sized dummy with a .50 cal, and the dummy was wearing armor plate to ensure that it absorbed all the energy of the bullet. even so, the dummy was only knocked back a few inches. You needed a high-speed camera to see the motion.
Yep.

I have shot at pumpkins with my .50 BMG. The pumpkins were just sitting on top of some RR ties. They didn't budge at all. And the pumpkins didn't explode... the bullets simply left a clean, half-inch diameter holes through the pumpkins. :(

So I was kinda disappointed that the .50 didn't obliterate the pumpkins. But then I discovered something that would... a 12 gage shotgun. A 12 gage slug hitting a pumpkin is much more impressive than a .50 BMG round. Pumpkin guts flew everywhere. :cool:

[MrJackboots]

Hydrostatic shock resulting in persistent trauma doesn't occur in rounds moving less than ~1500 fps. Hydrostatic shock can be an issue with centerfire rifle, but not standard pistol trauma.

I didn't know this exact figure, but that probably accounts for the famous effectiveness of the .357 Magnum. DoubleTap's 125-grain load (http://www.doubletapammo.com/php/catalog/product_info.php?cPath=21_27&products_id=48) gets about 1750 fps out of a 6" barrel, and their 158-grain (http://www.doubletapammo.com/php/catalog/product_info.php?cPath=21_27&products_id=49) gets about 1540.

DoubleTap is some high-performance ammunition, though.

[Alex_Dubinsky]

But what about if you have a machine gun and can spray multiple bullets into the target? That would provide a constant force.

[Don't Call Me Shirley]

2. If the round had enough power to knock the target down it would have to knock the shooter down as well given Newton's second law.
Actually, that is Newton's third law. For every action there is an equal and opposite reaction.

[Kinthalis]

Actually, that is Newton's third law. For every action there is an equal and opposite reaction.

Oops yes, I was copying from the other forum.

So the answer is no, which given everyone's input here makes perfect sense now.

In the game, BTW, the guns automatically calculate distance to target, atmospheric pressure, weather conditions, etc and then shaves off an appropriately sized round from a stock of metal and fires it. :)

The hammer rounds were explained as being designed to mushroom out, thereby imparting maximum kinetic energy into the target. But even so, as you all point out knocking down the target is a no go unless the tround continues to accelerate after being fired.

Thanks a bunch guys!

[Chronos]

But what about if you have a machine gun and can spray multiple bullets into the target? That would provide a constant force.To both the shooter and the target. The symmetry still holds.

[puppygod]

The hammer rounds were explained as being designed to mushroom out, thereby imparting maximum kinetic energy into the target. But even so, as you all point out knocking down the target is a no go unless the tround continues to accelerate after being fired.

If that's how they explained it, then knocking down doesn't makes sense. I've seen somewhere good comparison. It was along the lines of total energy of .45 bullet equal to 1 pound brick dropped from three inches. I don't remember equal values and am too lazy to calculate it now, but it was something around that order of magnitude. Hardly capable of knocking anybody down, even with mushrooming, 100% "energy transfer" or other exotic explanations.

[Chronos]

Actually, mushrooming will tend to cause a higher efficiency of energy transfer. If you transfer anything less than 100% of the bullet's energy to the target, the bullet is going to come out the other side (or bounce off, but that's unlikely for human targets). If the bullet mushrooms inside of the target, it decreases the chance of it coming out the other side. Thus, mushrooming will increase the efficiency of energy transfer.

It still won't knock the target over, because most of the energy that gets transfered ends up as heat, not kinetic energy.

[Alex_Dubinsky]

The hammer rounds were explained as being designed to mushroom out, thereby imparting maximum kinetic energy into the target. But even so, as you all point out knocking down the target is a no go unless the tround continues to accelerate after being fired.People here are being overly generalist and taking the average too many times. If a really big, strong guy properly stands as he fires a very large caliber round that does mushroom out to tranfer its momentum (and that does actually make a difference) then it really could knock back an average-sized person or even someone his own weight. Any martial artist or football player will tell you that the third law doesn't make such things impossible. In fact, if you ever did try to design a weapon that'd do it, then a large "hammer" traveling at a relatively low velocity that deforms at impact would be the way to do it. Points to whoever thought of your game.

[Oslo Ostragoth]

Incorrect.

I have a .50 BMG rifle. Serbu BFG-50. I shoot it in our backyard. (Seriously.) Just last Saturday I was shooting it in the offhand (standing) position, just for fun. It didn't lift me or throw me back. I just stood there and shot it like a normal rifle (albeit a lot heavier!). It was like shooting a 12 gage shotgun. And I'm not big... I'm only 5' 6".

In case you don't believe me, would you like me to videotape it and post it on youtube?The hell you say. I demand that you provide video.

;)

[Airman Doors, USAF]

Actually, mushrooming will tend to cause a higher efficiency of energy transfer. If you transfer anything less than 100% of the bullet's energy to the target, the bullet is going to come out the other side (or bounce off, but that's unlikely for human targets). If the bullet mushrooms inside of the target, it decreases the chance of it coming out the other side. Thus, mushrooming will increase the efficiency of energy transfer.

It still won't knock the target over, because most of the energy that gets transfered ends up as heat, not kinetic energy.

Not to hijack this thread, but that brings up an interesting point: back in 1993, Black Talon hollowpoint bullets were called "cop killer" bullets and "armor piercing" bullets, very nearly leading to the banning of hollowpoint bullets. Ironically, because they do mushroom, they were not prone to overpenetration and certainly were not capable of piercing armor.

Just an interesting historical tidbit that is somewhat relevant to the topic, if not entirely on point.