Cancelling an Equal and Opposite Reaction - Gun Recoil

[HyperSpeed]

The problem soon discussed has my mind reeling. (You will soon clearly see that I lack an engineering degree!)

Some time ago I became fascinated by the giant Mechem NTW-20 anti-material rifle. While it is not the only man-portable anti-material that exists, the ones that do exist seem to max out near the 20x110mm cartridge size and energy levels. The Mechem uses a recoiling barrel and action, with what appears to be a hydraulic shock/damper below the barrel.

Rifles such as that one have got me thinking about just how big of a cartridge might be possible to fire from a large rifle format which is not actually bolted down--if the right means of recoil management were employed beyond simple addition of system weight total. Naturally, the first recoil mitigating devices which come to mind would be similar to the NTW-20's; some kind of shock/damper system with a tunable bound rate to change the deceleration profile of the force.

Then I got to thinking about something different. Semi-auto rifles often use a port at some point down the barrel to scavenge barrel pressure which is driving the projectile, to push a piston-rod back which unlocks and opens the bolt with enough force to slam it rearward quickly, eject, and feed the next round into the chamber. Well, what would happen if a similar concept to that was used with a very large caliber round (where a lot of gas volume was available), but instead of driving a piston backwards, a piston and weight was driven forward, and the weight would slam forward as the gun was moving rearward--would this actually reduce the recoil impulse total?

This is what I mean when I say my mind reels at this problem, because the problem going on in my mind is the problem of action and equal and opposite reaction. I know that the same force is applied to the projectile and to the rifle when a shot is fired, but the energy bias moves into the projectile because of the difference in their weights. If the projectile weighed the same as the rifle, they would both travel apart with the same speed and energy when the shot was fired. Ok, so how do we go about calculating this event if combustion energy was used to move a weight forward? Would it be possible to create an energy bias in the system so that the weight could negate the rifles recoil, eventually, or would the excess rifle recoil created while the weight was moved forward equal the weight's own potential to reverse any recoil at all? Hence the equal opposite reaction, in other words. Something tells me the latter of things is probably true, or I would imagine that "recoilless" rifles using weights would probably be everywhere firing huge rounds like 30x173mm (Tim the Toolman Taylor grunts!). I thought that just maybe, through some difference in system timing the events could be used to possibly offset one another in periodic steps of events, but I just can't wrap my head around it without knowing the mathematical breakdown of events at play for certain.

P.S. When I input a sufficient weight moved at a rather slow speed compared to the bullet in a recoil calculator, the energy which is given to the weight indeed provides a substantial energy difference from the recoil energy generated into the rifle--just like the projectile has far more energy than the rifle recoil does. So it does indeed look like this could potentially be a working system of recoil reduction. Hmmm...

Ok, I'm all ears and popcorn now.

 

[HHaase]

Oh, recoilless rifles are a FUN topic. They got a hell of a lot bigger than 30mm, and they've been around a long time. The most common versions work via venting a portion of the explosive gasses to the rear, not for thrust but more as a counter-mass. It's a surprisingly effective concept as long as you don't stand behind it. The "Davis Gun" was the first, back in WW1, mounting a 3" gun on a lightweight biplane for anti-sub work.

The US has fielded in them in all sorts of sizes and mountings since WW2, though the heavier ones were eventually replaced by missiles, as the rifles were just a lot bigger and bulkier to do the same job as a missile system. The Carl Gustaf is still a commonly fielded, shoulder fired, 84mm recoilless rifle, of which the AT-4 is a single-shot derivative. Essentially the modern equivalent of the Bazooka.

 

[Funkworks]

piston and weight was driven forward, and the weight would slam forward as the gun was moving rearward--would this actually reduce the recoil impulse total?

I see a gun shot as a 1-dimensional explosion (much like rocket parachute ejection) and I think of it in terms of momentum: mv (mass x velocity). The purpose is to provide as much mv to the projectile as possible. Momentum mv in the opposite direction is recoil, but it can be mitigated with vents and-or damping.

If you’re going to take some forward momentum and use it to move a piston instead of the projectile, it seems a little counterproductive. Sure you could reduce the recoil, but you’ll also have a slower projectile, so what’s the point. Might as well just use a smaller, cheaper gun.

I say this as someone who taught college physics, not a rifle engineer.

 

[FishnBeer]

I have thought about this before, a recoil reducing device that basically fires in the opposite direction to cancel recoil. I pondered on pros and cons. If it is a shoulder fired rifle you would be adding quite a bit of mass and would be diffucult to carry. Even if it was a stationary weapon, it would need to be rather bulky with that much energy going in all directions.

The energy transfer may affect accuracy. If you have ever watched slow motion shots of an AK type rifle firing, the barrel deflects quite a bit. That is not just from the propellant burning and pushing the projectile out the muzzle, but also the gas system pushing on a large piston that racks the bolt rearward against spring tension. If you scale up, I would imagine the only way to counteract that deflection would be to add more mass to stressed areas for strength. Not to mention, when the moving mass that is intended to cancel out recoil reaches the end of its stroke, it will upset the balance of the design likely resulting in muzzle rise.

The only design I thought would work would be a cross between a blowback system on a pistol and a bolt carrier that would move in the opposite direction. Think of a AR pattern rifle with no gas system and add a bunch of weight to the bolt carrier.

I feel like John Browning thought about all this before while sitting on the can.

 

[Sooner Boomer]

kinetic energy = 1/2 mass times velocity squared

kinetic energy in bullet = kinetic energy in recoil (equal but opposite effects)

What's important to note is that the energy goes up with the velocity squared. Velocity has a lot more effect than mass.

What I find fascinating are recoilless rifles. My favorite is the Bofors Carl Gustav. It's an 84mm anti-tank, anti-materiel weapon. The newest version is a little over 3 feet long. This is a true recoilless, ejecting a high velocity blast out the back of the weapon as the projectile goes downrange on a ballistic path. There are hybrids like the RPG-7 which uses a recoiless-like "booster" charge to launch the round away from the user before the rocket engine ignites. Almost all recoilless weapons will have a significant back blast!

 

[Zeus-cat]

...
Then I got to thinking about something different. Semi-auto rifles often use a port at some point down the barrel to scavenge barrel pressure which is driving the projectile, to push a piston-rod back which unlocks and opens the bolt with enough force to slam it rearward quickly, eject, and feed the next round into the chamber. Well, what would happen if a similar concept to that was used with a very large caliber round (where a lot of gas volume was available), but instead of driving a piston backwards, a piston and weight was driven forward, and the weight would slam forward as the gun was moving rearward--would this actually reduce the recoil impulse total?
...

I think you have designed a system that gives you the worst of all possible outcomes.

First, you are venting some of the gas generated to move a weight so that is less gas to push the projectile forward. Second, the system you describe will have some reaction time that could be longer than the time the projectile is inside the barrel. So, the reduction in recoil by slamming your weight against the front of the gun doesn't occur until after the projectile has left the barrel (I could be wrong on that based on how fast the weight moves, distance travelled, etc.).

The energy to move the projectile forward has to be countered by energy pushing back on the gun. PERIOD. You can absorb the energy into your shoulder. You can bolt the gun to a large object and let the object take the recoil. Or you can use a spade at the back of a cannon and put the energy into the earth and add/subtract from the earth's rotational energy by a ridiculously miniscule amount.

 

[HHaase]

I have thought about this before, a recoil reducing device that basically fires in the opposite direction to cancel recoil. I pondered on pros and cons. If it is a shoulder fired rifle you would be adding quite a bit of mass and would be diffucult to carry. Even if it was a stationary weapon, it would need to be rather bulky with that much energy going in all directions.

The KRISS vector pistol uses an odd recoil mechanism that angles the breechblock downward into the hand grip, which to an extent helps to counteract muzzle rise. Really wonky looking system to be honest.

 

[Jeff Lassahn]

kinetic energy in bullet = kinetic energy in recoil (equal but opposite effects)

This isn't quite correct.
It's momentum (mass times velocity) that must be equal and opposite, not kinetic energy. A good design will use this to its advantage, trying to give as large a fraction of the energy to the projectile as possible.

 

[Steve Shannon]

This isn't quite correct.
It's momentum (mass times velocity) that must be equal and opposite, not kinetic energy. A good design will use this to its advantage, trying to give as large a fraction of the energy to the projectile as possible.

Exactly! Momentum must always be conserved.

And to the OP’s question, if I understand the question correctly, using the “excess” gas to force additional mass forward will not reduce recoil. It will have the opposite effect. A good real world example is launching a rifle grenade. No matter what, momentum forward must equal momentum rearward.

 

[Jeff Lassahn]

When the projectile is fired forward, you have to have something move backwards to conserve momentum.
You've basically got three choices:
1. send something light backwards at high speed (like gas from a rocket exhaust). This is inefficient because a lot of energy goes into the backward moving thing, but might be desirable for other reasons.
2. send something heavy backwards at low speed (more efficient, but you have to deal with your gun dumping large objects out the back)
3. use the person holding the gun (or some other fixed object) as the thing that moves backwards. This kinda means it's not really a recoilless gun anymore, but you can try playing games with springs or whatever to distribute the forces in a way that's less annoying.

 

[DES]

When the projectile is fired forward, you have to have something move backwards to conserve momentum.
You've basically got three choices:
1. send something light backwards at high speed (like gas from a rocket exhaust). This is inefficient because a lot of energy goes into the backward moving thing, but might be desirable for other reasons.
2. send something heavy backwards at low speed (more efficient, but you have to deal with your gun dumping large objects out the back)
3. use the person holding the gun (or some other fixed object) as the thing that moves backwards. This kinda means it's not really a recoilless gun anymore, but you can try playing games with springs or whatever to distribute the forces in a way that's less annoying.

#2 is actually how most gas operated rifles function. The bolt (heavy) is unlocked and started backwards while the burnt propellant gas is still exhausting the barrel. The propellant gunpower is around 30% of the total mass exiting the muzzle; by using that to propel the bolt rapidly backwards at the same time the gas is exhausting, total recoil is reduced. My M1 Garand 30-06 has a lot less recoil than my bolt action 30-06.

 

[rharshberger]

I am using a small scope for my AK-47. I wanted to have the best AK-47 scope, so before buying it, I searched for a bigger scope, but I have read on different forums that there is no need for a big scope on such rifles.

The AK has definite practical range limitations and is not known for accuracy beyond "minute of human"... so a 3X scope is usually plenty, but that discussion is something for the Guns & Rockets thread.

 

[boomtube-mk2]

 

[MidOH]

Remove your piston thingy, and you basically described a plain Olde muzzle brake or compensator. But yes, you can send gas from the barrel, backwards to produce thrust.

"But it wastes gas and reduces performance''. Then add more powder.

"But too much powder will kaboom''. Then use more powder that's slower burning. That will reduce pressure on the case, since the bullet will be gone, dropping the pressure, before the case sees excessive pressure. But that leftover unburnt powder can still apply thrust to the comp, brake, or rearward whatever.

Research USPSA open division. And recoiless rifles. M67. M40.