Physics of shooting a rifle

[tangolima]

Backing up a bit to shooting a man flying off his feet. Theoretically I do think it is possible (not impossible).

A rifle bullet can easily have kinetic energy of 1,000 ft-lb. It is enough to send a 1000 lb mass 1 foot straight up, or to send 200 lb mass moving at 18 fps (about 12mph). But in practice, it is rather difficult to get that energy fully transferred to the target.

One story account is the closest that I have heard. It was on a documentary. An army medic was hit by a sniper round. The chest plate in his body armor saved him. It was slightly nicked and they couldn't recovered the projectile. It was believed that the bullet bounced back upon impact. Basically his body absorbed the substantial portion of the energy. He remembered it was like being ramped by a fast moving truck. When he regained his bearings, he was on the ground flat on his back meters away from where he was standing. His teammates' quick reaction and fierce covering fire kept the enemy from follow-up shots. They managed to pull him to safety. Nobody remembered seeing him flying in air, but I would think it must be quite close to.

-TL

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[JohnKSa]

Mythbusters did a test with a crash test dummy. They set it up with an AR500 plate in its chest.

They shot it with a .50BMG at arm's length distance. The bullet did stop in the dummy. It went through the plate but stopped in the metal spine.

The dummy was not moved backwards by an appreciable amount.

Here's more on that test, including some screen captures from the test.

https://thefiringline.com/forums/showpost.php?p=6039165&postcount=15

The problem is that there's all kinds of energy used up doing things other than moving the target. I've seen slow motion footage where a bullet impact on steel causes the steel to splash like water and/or the bullet to deform like a marshmallow. That's energy that's no longer available to push the target backwards because it was used to do something else.

Here's a post with a link to a guy with a ballistics vest getting shot with a .308 rifle at close range. He stands on one foot and the rifle still doesn't knock him down.

There's another link showing rifle testing using a live person wearing a bulletproof vest.

https://thefiringline.com/forums/showpost.php?p=6574540&postcount=71

Even with the bullet stopping in the target, even when the bullet is from a .50BMG there's no significant push backwards.

Remember, momentum transfer requires inelastic collision. Once things start deforming appreciably there's no way to get an accurate picture of what's going on from simple momentum calculations.

 

[tangolima]

It is indeed very difficult to set up just right to make it happen. But proving impossibility itself is an impossibility. The usual proof is a quote to Newton's 3rd law, claiming it is physically impossible.

Both elastic and inelastic collisions follow principle of conservation of momentum. They transfer momentum and energy alright, just by different amount. A bug smashed on windshield is inelastic collision. A baseball bat landing on a skull is elastic collision. Both hurt like hell.

-TL

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[JohnKSa]

But proving impossibility itself is an impossibility.

Proving a negative, depending on the specific negative can be impossible.

For example proving that a particular person never said a particular quote is impossible because it's not possible to get an exhaustive list of everything any person ever said. On the other hand, proving that the person never wrote the quote in one of their published books could be easy if one had access to all the books that the person published.

Proving a .308 rifle bullet can't send a person flying is certainly possible.

Set up a scenario involving a cartridge with tremendously more energy/momentum than the .308 cartridge can provide. Make sure that in that scenario, the target is similar to a human in shape and weight but is designed so that it absorbs all of the momentum/energy of the impact.

If that scenario doesn't result in the target being sent flying then it's pretty clear that using less energy/momentum can't possibly impart more energy/momentum to the target.

Both elastic and inelastic collisions follow principle of conservation of momentum.

The way I stated it was incorrect. What I was getting at was this:

Shoot a water balloon with a bullet. If conservation of momentum is followed in the conventional manner to determine the outcome, then the water balloon will be driven in the direction of bullet travel with a velocity that can be calculated by the momentum transfer equation.

Clearly that is not the case. In fact, most of the mass that once made up the water balloon will travel in directions other than the direction of the bullet impact. Some of it will actually travel in a direction almost directly opposite the direction of the bullet impact, splashing the shooter if the distance is short enough.

Momentum is conserved, but not in any sense that is useful for this discussion.

https://www.youtube.com/watch?v=5b1lDyBwa_0

Check out the egg being shot at 4:33 in this video.

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

Even if the balloon doesn't burst, say it's made up of a gel that is self-healing and stretches to any length without breaking, the bullet impact still won't drive it downrange to any significant extent because the impact energy will be used expanding the ball out in all directions. It will move overall a little bit in the process of stretching and snapping back, but not anything like the momentum transfer equation will predict.

 

[mehavey]

I think the endless YouTube videos of gel blocks -- even "skeletonized" gel blocks -- not going anywhere in particular is fairly conclusive.

 

[Hawg]

I have read that a .58 minie ball has twice the knockdown power of a 30-06. It has been documented that soldiers shot during the civil war with .58's fell forward.

 

[tangolima]

Physics is study of energy transfer. When there is enough energy, it is possible. The question is how to direct the energy to the right place. I'm holding on to the hope. Experiments fail to make it happen. It is just that. They fail to make it happen. Not a proof to the impossibility.

Before this discussion cork screws into a philosophical debate. Let's go back to muzzle climb.

Shoulder on the butt stock. Average force on the stock 50lbf for 2ms. Center of butt stock is 5" below bore axis. The torque to raise the muzzle is 250 inch-lb. It is being countered by the weight of the rifle. The CG is usually right at the chamber about 24" from the butt stock. 7.5lb gives rise a torque of 180 inch-lb. The net torque to rotate the rifle is 70 inch-lb or about 6 ft-lb.

It doesn't take much to significantly change POI. For a 40" long rifle, the muzzle just needs to rise by 0.01" to move POI by 1" at 100yd.

There are actually ways to totally eliminate muzzle rise. The above calculation gives a few hints.

-TL

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[stagpanther]

Grip on the foreend as well as the pull in would counteract some of that muzzle rise, I guess it really depends on how much rise there before the bullet clears the muzzle. The big magnums I shoot--especially ones without a muzzle device--repressing that muzzle lift is going to be hard to do (and transfer more felt recoil). It seems to me the muzzle lift is just another vector of recoil force resulting from a lever moment due to the off-centerline contact point of the shoulder.

 

[JohnKSa]

Physics is study of energy transfer. When there is enough energy, it is possible. The question is how to direct the energy to the right place.

The answer is that even when using tremendously more energy than conventional hunting rifle rounds, there's still just not enough energy applied in such a way as to make it happen. The problem is that a huge amount of the energy is used up doing things other than moving the target downrange. Any bullet deformation, any noise from the impact, any heat from the impact, any deformation of the target, any force applied other than directly downrange, all of that uses up energy. Just like the water balloon, there's a lot of motion at impact, but very little of the resulting motion is actually in the downrange direction.

Experiments fail to make it happen. It is just that. They fail to make it happen. Not a proof to the impossibility.

If experiments using projectiles with far more energy and momentum than conventional rifle rounds, hitting targets that deform/yield far less than human tissue still fail to make it happen, what possible reason could there be to expect that far less energy and momentum with a far more yielding target could make it happen?

If 1000 + 1000 << desired result then how could 50 + 50 somehow provide the desired result?

Go shoot a prairie dog with a .338 Lapua and see what happens. It will get blown apart, but pieces will go in all directions with a center hit. Very little of the prairie dog will get moved directly downrange. Look at the old bowling pin matches. Even with high-momentum bullets, it was not a simple task to knock the bowling pins back off the edge of the tables and they are much tougher than flesh and blood and far, far smaller and lighter than humans.

Look at the results of the crash test dummy shot with a .50BMG at arm's length without the bullet exiting the dummy. It hardly moved backwards at all.

Physics is about observation, not about making wishes come true. Science is looking at the outcomes of experiments and accepting the reality of what they tell you.

There are actually ways to totally eliminate muzzle rise.

The Russians came up with a machinegun that did it. The action reciprocated straight backwards so that the next round was fired with no muzzle rise. Eventually the muzzle rose, but not appreciably until the 3 round burst was complete.

So that's one way, set up the action/barrel to reciprocate straight backwards until the bullet is out of the barrel.

A lot of recoil operated autopistols function like this. The coupling of the slide/barrel to the frame is loose enough during the period that the bullet is in the barrel that there is very little muzzle rise. Not until the unlocking process is significant recoil force first applied to the frame resulting in muzzle rise and, by design, the bullet is out of the barrel by then. Revolvers are much different. Since the coupling of the barrel to the frame is completely rigid, muzzle rise begins the instant that recoil motion starts.

Another way is to set up the stock and action so that the geometry results in recoil straight back into the shooter's shoulder so that the muzzle doesn't rise appreciably due to torque. The Chiappa revolver is an example of a design that tries to alter the relationship of the bore axis to the point where recoil is resisted by the shooter in such a way as to minimize muzzle rise.

 

[stagpanther]

Hmmm...I've seen videos of animals doing flips and wild jumps upon projectile impact--could be autonomic nerve system shock response?

 

[JohnKSa]

Right. Animals have the ability to move their bodies voluntarily or reflexively. If someone is stuck with a pin and jumps into the air, we don't attribute that motion to the energy imparted by the pin because we can stick inanimate objects with a pin and they don't move.

Same with bullets. The bullet strike may cause an animal to react by jumping/flipping, but unless the animal is very small and the bullet is quite energetic, that's a reaction, not the actual bullet imparting sufficient momentum to drive the animal into the air.

 

[jrothWA]

We have TOO much time, on our hands!

Should have used SI and drop some calculations.

 

[JohnKSa]

Yes, it's simpler in that unit system, but people seem more comfortable talking about pounds and foot pounds than about joules and Newtons.

 

[tangolima]

SI at school and imperial at work. I can go back and forth. Like telling the same story in different languages. None is better than the other.

Upward torque to raise the muzzle, and downward torque vise versa. No force on the stock butt or no distance between the barrel's axis and the center of the stock, there will be no upward torque. Theoretically possible but neither is practical. There will always be such upward torque the way we shoot. However the weight of the rifle and the position of the CG gives rise the ever present downward torque. As long as the upward torque doesn't exceed the downward torque, the muzzle will not rise during the barrel time. Note that the force between the stock and shoulder is never constant. It peaks after the bullet exits the muzzle so the muzzle may still climb. But that doesn't affect POI.

Not leaning too hard on the stock reduces the said force. Gun designs with low bore axis, such as AR, help shorten the moment arm. Long range shooters added weight to the forend to increase the weight and to move the CG away from the shoulder. Of course extra downward torque can be applied to the rifle by...slinging up tight.

Those are examples of methods to eliminate muzzle climb. There are a few more.

BTW, the force can really go to zero if the rifle is let free recoil. The ransom mega 2 rifle rest is such device. This leads to another rabbit hole for later.

-TL

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[tangolima]

We learned at gunsmithing school that American and European shoot their rifles differently. American firmly grabs hold of the forend, whereas the European rests the forend on their palm. One tries to eliminate or reduce the muzzle climb, and the other just lets the recoil do its thing. I asked my European coworkers, French, Finnish, Swedish, British etc, about it but it was inconclusive. They didn't remember exactly as they hadn't thought much about it.

My shooting is mostly on front rest and rear bag, either on bench or prone, with my left hand squeezing the rear bag. Effectively it is "European". The rifles I shoot are mostly milsurps with medium weight and steel plate on heel of the butt stock. POI elevation varies quite noticeably with loads. I'm pretty sure muzzle climb due to recoil plays a significant role.

I have been considering letting go of the rear bag and holding the forend with my left hand, pulling the rifle down on to the front rest. It will reduce, if not eliminate, the muzzle climb. I will give it a serious try.

-TL

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[mehavey]

High/mass recoil and/or snapshot -- Forearm hold
Offhand at targets -- from 5.56 through Garand -- flat palm/hand under forearm at/near balance point.
`59 Sharps and other carbines? Forearm hold.

Hunting? Any support I can get.

 

[tangolima]

The strongest kicking caliber I load and shoot is 8mm Mauser, which is really not bad. So I rest the forend on palm for offhand. POI elevation does change quite a bit if I change the load or my posture. Never really like slinging up. Maybe I should give it a serious try. Matching-winning-record-breaking people do that, do they not?

There is another very effective method to eliminate muzzle. Didn't know that till I started using bipods. Friend competes in PRS (precision rifle series). He demonstrated that with proper pre-loading the bipod, there could be no muzzle climb (they call that bipod hop) at all. I tried that and it was true. I couldn't understand how pushing the rifle forward could also keep it down. He has no clue either, only knows that it works. I finally figured out the mechanism. It is the downward torque created by the preloading force and the height of bipod.

That really got me thinking. I didn't care about bipod and vertical forend grip, as they are too tacticool. Now I want to put one on each of my rifles. Well one thing to watch out, only rifles with free floating barrel, or the cons will out weigh the pros.

-TL

PS I repaired a couple of 12 gauge auto shotguns with forends broken off. Shooters all held on to the forend tight while firing magnum rounds. The wood got torn off. I glued it back with epoxy. I had the same talk with owners to persuade them to perhaps let the recoil do its thing. I shouldn't mind the repeated business.

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[tangolima]

One more rabbit hole. Will stop if there is no interest. Don't intend to have monologue.

The recoil force is between the heel of the butt stock and the shooter's shoulder. It can be tapped off to cycle the action on auto loaders, as JohnKSa described in his post. Such designs are called short-recoil or long-recoil. Such designs, although simple, have one drawback. It can be limp-wristed or limp-shouldered, that the recoil force is inadequate to complete the cycling of the action.

Gas operated action is another kind of auto loading. Can gas operated actions be limp-wristed or limp-shouldered?

-TL

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[Bart B.]

I've coached team matches where 4 people shoot the same rifle and ammo. A couple MOA spread in sight zeros was normal. Both bolt action and semiautomatic.

 

[JohnKSa]

The recoil force is between the heel of the butt stock and the shooter's shoulder. It can be tapped off to cycle the action on auto loaders, as JohnKSa described in his post.

There are also actions, like Benelli employs in some of their shotguns, that exploit the inertia of a heavy bolt vs the force of acceleration of the rest of the action/firearm generated by recoil. That acceleration of recoil compresses a spring against the inertia of the heavy bolt and when the acceleration of recoil stops (and the force also therefore stops--remember zero acceleration means zero force) the spring then expands to operate the bolt and perform the extraction/ejection operations cycle with a conventional recoil spring in the stock then returning the bolt to battery, feeding a new shell in the process.

We know that conventional recoil operated systems work better the more rigidly the action is restrained from recoiling and will tend to malfunction if insufficient constraint is applied with a loose hold.

So does an action like the inertial action cycle more effectively when the firearm is rigidly constrained?