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Mercury vs FPS


Cash Duckett

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Ok, if a mercury recoil management system reduces (felt) recoil, what does it do (if anything) to the velocity? Wouldn't this be like "limp wristing" a 1911? I've often wondered if there was a measurable difference between velocity of round shot with a "soft" shoulder, (one that was easily moved backwards), and a "hard" shoulder, (more like putting the buttstock against something solid). I've never used any recoil system, but the way I understand physics, there would be less velocity with a recoil system. In fact, I would think that the better the system, the less the bullit velocity. I also think that adding lead, that doesn't move around, would reduce recoil without losing velocity. Anyone done this test?

Cash

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I don't think so. We start with a fixed amount of momentum in either direction. ANY recoil management system slows the recoil rate, but does not reduce the total amount of energy (momentum), either of the shot or the gun. A Mercury recoil reducer simply "folds back" the momentum onto itself as it were. When the stock tries to move rearward, it sloshes the mercury, which in turn slops against the front of the reducer body, "braking" the stock's rearward momentum some, but in no way changing the total momentum created when the charge ignites, or the amount applied to the shot itself.

 

Limp wristing a 1911 doesn't change the bullet velocity, it changes the RELATIVE velocity between the slide and the frame, as instead of the frame being held somewhat still, the lightly held gun tries to "go with" the slide, but that still means exactly half of the momentum goes rearward, and half is applied to the bullet. Firmly grasping the gun reduced the proportion of that rearward energy transferred to the frame, allowing a greater SLIDE motion, which is needed to overcome the s[ring and correctly cycle the slide. None of this materially changes the amount of momentum applied to the bullet. Sure if you start out with more momentum (hotter load), ou can get away with a less firm grip because even at a somewhat reduced ratio of slide to frame momentum, sufficient SLIDE velocity remains to overcome the spring, but it isn't as if your grip changes the velocity of the bullet.

 

Hint: The 1911 barrel is held in lock-up by the forward tension of the BULLET trying to take the barrel with it as it exits forward. The barrel is relesed and the now unlocked slide allowed to recoil ONLY after that forward pull is insufficient to counter the effects of recoil (some debate exists as to exactly when that occurs, but it's just about as the bullet leaves the muzzle, give or take a few nano-seconds. The slide doesn't even start to move until the barrel drops out of lockup, at which time the bullet is just about out of the muzzle. So no change in relative momentum applied to the slide or the frame because of limp wristing can change bullet velocity. Slide velocity relative to the frame yes, but not bullet velocity relative to the world.

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Adding a recoil reducer would increase the mass of the gun, slowing the velocity of the gun in recoil and reduce the felt recoil. Since the gun's recoil velocity is reduced, the energy which must be conserved according to Newton's second law, is transferred to the shot. The end result would be an increase in muzzle velocity. (It probably would be very small though.)

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Additionally, "limp wristing" a 1911 only applies to greatly reduced loads, such as those used in bulls eye shooting. With "standard" loads, a 1911 will function when held with the thumb and index finger only, recoiling freely without any other support.

Recoil reduction systems have no effect on bullet velocity. As explained by AJ.

 

Coffinmaker

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Affect velocity?? Not a chance !

 

RBK

 

 

A BIG +1 !

 

No way can a recoil reducer affect velocity! (neither do sound suppressors, by the way)

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I think we're treading the fine line between interior and exterior ballistics.

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We are indeed muddied by the inter VS exterior ballistics. With anything short of a large naval gun, MOST of the recoil of the stock happens AFTER the shot has reached max velocity, AND any changing of the gun, either through weight increase or addition of any mechanical, yhydraulic or mervury reducer only SLOWS momentum of the stock, it doesn't truely reduce it at all.

 

The simple waay to put numbers to it is this.

 

If the entire charge produces X amount of linear thrust, 1/2 X goes forward, pushing the bullet, 1/2 x goes backward, pushing the gun. NO MATTER WHAT WE DO, from tying a railroad tie to the stock to cutting the stock off and holding the receiver in our hands, exacttly 1/2 X rearward force always exists, and 1/2 X is left to push the bullet forward. Now there are some parasitic losses, drag down the bore, crushing of wads increasing the drag, etc, but the FORWARD motion is always produced by exactly the same force as the rearward motion (by definition).

 

IF we were to put the gun on rails and use light, long springs to push a heavy sled rearward, we cpuld theoretically stretch out the TIME used to expend that energy so much it'd be really slow and painless. If we simply held the gun against our body and our back to the wall, maybe use a fixture to prevent any muzzle rise, we'd get ALL tthe energy right now, and nobody is gonna like it. BUT, it's still the same amount of energy, and has zero effect on forward flight except that any muzzle rise before the shot clears the barrel will effect POI. Heavier guns push back more slowly, light guns more quickly, but how MUCH they push is a direct result of bullet (shot) weight and charge (velocity).

 

The mental mistake is similar to those we make with electricity or even water flow. HOW MUCH energy does a gallon of water dropped one foot impart upon you? Let's say it's X. Does it matter if it's a balloon full all at once, or a leaky tank dripping over a whole day? NOT AT ALL. The energy is equal to the weight of the water and it's velocity, however FELT results are inversely proportional to time..... And so it is with aming a shotgun. Anything you can do to spread that energy out over a longer time reduces the felt recoil, while not changing the actual energy at all. If you can manage to "fold" a bit of that energy around and use it against itself As a brake (which is what mercury recoil reducers do), YOUR BODY takes less of a hit, but the energy produced by the shot is exactly the same.....

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We are indeed muddied by the inter VS exterior ballistics. With anything short of a large naval gun, MOST of the recoil of the stock happens AFTER the shot has reached max velocity, AND any changing of the gun, either through weight increase or addition of any mechanical, yhydraulic or mervury reducer only SLOWS momentum of the stock, it doesn't truely reduce it at all.

 

The simple waay to put numbers to it is this.

 

If the entire charge produces X amount of linear thrust, 1/2 X goes forward, pushing the bullet, 1/2 x goes backward, pushing the gun. NO MATTER WHAT WE DO, from tying a railroad tie to the stock to cutting the stock off and holding the receiver in our hands, exacttly 1/2 X rearward force always exists, and 1/2 X is left to push the bullet forward. Now there are some parasitic losses, drag down the bore, crushing of wads increasing the drag, etc, but the FORWARD motion is always produced by exactly the same force as the rearward motion (by definition).

 

IF we were to put the gun on rails and use light, long springs to push a heavy sled rearward, we cpuld theoretically stretch out the TIME used to expend that energy so much it'd be really slow and painless. If we simply held the gun against our body and our back to the wall, maybe use a fixture to prevent any muzzle rise, we'd get ALL tthe energy right now, and nobody is gonna like it. BUT, it's still the same amount of energy, and has zero effect on forward flight except that any muzzle rise before the shot clears the barrel will effect POI. Heavier guns push back more slowly, light guns more quickly, but how MUCH they push is a direct result of bullet (shot) weight and charge (velocity).

 

The mental mistake is similar to those we make with electricity or even water flow. HOW MUCH energy does a gallon of water dropped one foot impart upon you? Let's say it's X. Does it matter if it's a balloon full all at once, or a leaky tank dripping over a whole day? NOT AT ALL. The energy is equal to the weight of the water and it's velocity, however FELT results are inversely proportional to time..... And so it is with aming a shotgun. Anything you can do to spread that energy out over a longer time reduces the felt recoil, while not changing the actual energy at all. If you can manage to "fold" a bit of that energy around and use it against itself As a brake (which is what mercury recoil reducers do), YOUR BODY takes less of a hit, but the energy produced by the shot is exactly the same.....

 

Thanks for taking the time to provide as very thorough and understandable reply. I learned alot from your reply and was reminded of some difficult days in physics class. Whooop!

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We are indeed muddied by the inter VS exterior ballistics. With anything short of a large naval gun, MOST of the recoil of the stock happens AFTER the shot has reached max velocity, AND any changing of the gun, either through weight increase or addition of any mechanical, yhydraulic or mervury reducer only SLOWS momentum of the stock, it doesn't truely reduce it at all.

 

 

Great explanation, but since we are being technical I have to correct one thing (sorry- it's the engineer in me).

"and any changing of the gun, either through weight increase or addition of any mechanical, hydraulic or mercury reducer only slows VELOCITY of the stock"

 

Due to the law of conservation of energy, you cannot slow the momentum. Momentum is energy (mass X velocity). As you said, the momentum of the projectile and gases will equal the momentum of the gun; therefore increasing the mass of the gun, will result in a decreased velocity of the gun.

 

Going one step further, the velocity of the gun is what affects perceived recoil. It is actually the change in velocity (going from motion to stop against your shoulder) that you feel as recoil. A change of velocity is acceleration (deceleration in this case). To decrease the acceleration, you can either decrease the velocity by increasing the mass, (remember the momentum equation from above) or by lengthening the time of the velocity change (this is what a recoil pad does).

 

Now that everybody has fallen asleep, I think that I will stop typing...

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Great explanation, but since we are being technical I have to correct one thing (sorry- it's the engineer in me).

"and any changing of the gun, either through weight increase or addition of any mechanical, hydraulic or mercury reducer only slows VELOCITY of the stock"

 

Due to the law of conservation of energy, you cannot slow the momentum. Momentum is energy (mass X velocity). As you said, the momentum of the projectile and gases will equal the momentum of the gun; therefore increasing the mass of the gun, will result in a decreased velocity of the gun.

 

Going one step further, the velocity of the gun is what affects perceived recoil. It is actually the change in velocity (going from motion to stop against your shoulder) that you feel as recoil. A change of velocity is acceleration (deceleration in this case). To decrease the acceleration, you can either decrease the velocity by increasing the mass, (remember the momentum equation from above) or by lengthening the time of the velocity change (this is what a recoil pad does).

 

Now that everybody has fallen asleep, I think that I will stop typing...

Frank, you are essentially correct of course, and your explanation is a perfect example of why engineers ahould never be allowed to teach middle school science (or write instructions). Unless your audience is a groups of fellow engineers, they're quite likely to go "Huh?"

 

Now chew on this. If we draw a graph where the area under the line represents the rearward action of the gun under recoil (momentum is a measure of quantity like gallons or watts, vellocity talks about the rate at which that energy is expended ), the horizontal line is time, and the vertical axis is velocity, the AREA under the line will always be constant with a given shot and charge, but BOTH the uphill (increasing velocity) and downhill (decreasing velocity) segments of the curve can be attenuated (flattened out some), as we stretch out the time needed to dissipate the forces, but the area under the line will not change..... Simply mounting a shotgun correctly, pulling it tight into the body, leaning into the shot and increasing the fraction of the shooter's weight on the forward foot will stretch out the time of the motion and flatten both ends of the curve, as we force the gun to rock the shooter along with itself, and mitigate it's ability to "stab" back violently. That's how silly shooters playing games CAN simultaneously touch off both barrels of a light coach gun using stout loads and not get battered to heck....

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