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Would changing the weight of gun components affect recoil?

  1. Dec 21, 2014 #1
    This question really does the need assistance of somebody trained in physical sciences. I've asked countless gun people and they are very opinionated but few of them can put forward answers that show they know anything about physical science.

    I myself dropped out of Engineering school halfway through, and 10 years ago I could have pulled out formulas and stuff and diagrammed this out, but it's been soo long, I just can't do it anymore. I'm getting confused looking at formulas for springs and stuff.

    I need to know if changing the weight of the moving parts of an AR15 can reduce recoil, or if the principal of conservation of energy overrides any changes in weight, whether lighter or heavier. The gas from the barrel of the AR15 travels down a tube from the barrel, and hits what's called the "gas key." The gas key is connected to the BCG (bolt carrier group.) When the gas hit's the BCG's gas key, that's what starts the rearward travel of the parts. Behind the BCG is a metal part about 3 inches long called a buffer. It's just a weight. The BCG and buffer are two seperate parts, but they act as a single unit when moving. Behind the buffer is a spring. The spring keeps constant pressure on the buffer which means the buffer is always in contact with the BCG. As the BCG and Buffer move backwards against the spring, eventually the BCG/Buffer will hit the very back of the tube that they're traveling which starts their return trip back into battery. I setup a system with a stronger spring and tuned gas port so that the BCG/Buffer never actually hit the back of the tube, but rather they're fully arrested by the spring's pressure before returning it all back into battery.

    My question is, if I change the weight of the BCG and/or Buffer, will that make any difference at all in felt recoil? Conservation of energy would tell me that if I doubled the weight it would just mean that the BCG/Buffer travel at half the speed but would still compress the spring just as far and impart just as much energy into the shooters shoulder. Conversely, if I cut the weight in half, it would just mean that the BCG/Buffer move at twice the speed but still compress the spring just as far and impart just as much energy into the shooters shoulder as if I did nothing at all. Is that correct? Or are there ways that cutting or increasing the weight might actually decrease felt recoil?

    For instance, right now I'm leaning towards cutting the weight would decrease felt recoil because since it's the same momentum in the system but travelling twice as fast, that means that the same amount of force is being applied to my shoulder but for half the time, right?

    Please keep in mind there are two different types of recoil that I'm aware of when shooting. The first is from the bullet and gasses traveling down the barrel and leaving the barrel. In a bolt action rifle, that would be the only recoil. But in a semi-automatic, the bolt parts start flying rearward creating recoil too. The latter is what I'm concerned with in this discussion.

    If somebody is interested in point of data. A light BCG/Buffer system would weigh approximately 11oz. The travel distance is 6 inches, and the cyclic rate is 800 rounds per minute if it were full auto. A heavy system would probably weigh 16 oz. I don't have any data for spring pressures or anything. And the above data points are best guesses.

    Here's the post I made at AR15.com that I need the answer to, please take a look at what the gun hobbyists are saying: http://www.ar15.com/forums/t_3_118/..._BCG_for_less_recoil__isn_t_it_the_same_.html

    If you don't know how an AR15 works and my description above wasn't helpful, here's a youtube video demonstrating the operation:
     
  2. jcsd
  3. Dec 21, 2014 #2

    Bystander

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    It will affect the time interval over which the momentum is transferred to your shoulder; longer time, less force.
     
  4. Dec 21, 2014 #3
    Okay, so was I correct that the same amount of energy is transferred no matter the weight of the BCG parts? In other words, if increase or decrease the weights but keep the spring the same, it should still travel the same distance down the spring right? It's just that the longer time absorbing that energy makes it feel lighter?
     
  5. Dec 21, 2014 #4

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    That's the engineering argument.
     
  6. Dec 21, 2014 #5
    I guess my next question if subjective, but if you're feeling that over 1/26th of a second (800rpm for a light bcg but only half the time is spent applying rearward pressure) versus 1/16th of a second (500rpm for a heavy bcg,) does it really matter? Would a person shoulder even notice since they're such quick impulses anyway? If it were a full auto weapon, it's obvious that the slower cyclic rate would reduce recoil because your feeling constant force for 2 to 3 second bursts. But a quick semi-auto pull of the trigger with a singular 1/26th or 1/16th second impulse would seem like it would be undetectable in difference.
     
  7. Dec 21, 2014 #6

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    Conservation of momentum: dp/dt = F on your shoulder; 1.6 x greater for the higher rate, given same projectile masses and velocities.
     
  8. Dec 21, 2014 #7
    Just for clarification, you're saying that the lighter/faster bolt has more recoil than the heavier/slower bolt?
     
  9. Dec 21, 2014 #8

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    For the same cartridge, both have the same recoil, if we define recoil as momentum you have to handle. The higher rate of fire means higher force against your shoulder.
     
  10. Dec 21, 2014 #9

    A.T.

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    Energy and momentum are two different things.
     
  11. Dec 21, 2014 #10

    sophiecentaur

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    No. That's not right. Only momentum is conserved in all collisions. Ignoring the fancy anti-recoil mechanisms, which cannot, in fact, alter that, the mass of projectile times its velocity equals the mass of the gun times its speed. The Kinetic Energy formula is mv2/2 so you can see that, with a massive gun, the v is proportionally less BUT the KE depends upon the v2, which will reduce the KE (and hence the share of the propellant energy). In the limit, a gun which is bolted solidly to the ground (or a huge mass) will not put any (measurable level of) energy into the gun + mounting.
    The recoil situation is another issue and, a fancy suspension can spread the momentum over a longer time interval- reducing the force on the shoulder. I guess they work by making sure the bullet has left the gun barrel before you have received the force of the impact on your shoulder. I have a feeling that anti recoil systems will actually reduce the energy available as KE of the bullet. (Someone can probably answer that for us.)
    A useful concept is Impulse, which is that total momentum change. Impulse is Force times time. Impulse is a great way to approach problems of collisions etc. and you really can't go wrong if you get the Impulse situation right. Energy is far more fickle and can easily let you down.
     
  12. Dec 21, 2014 #11

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    Depends on the system: gas operated automatic or semiautomatic loading systems parasitize projectile energy through gas bleeds, and when a recoil system is piggy-backed on top are net losers; muzzle brakes are generally parasitic (not universally); hydraulic systems for artillery, generally not.
     
  13. Dec 21, 2014 #12
    Yes that is true because of inertia if you have a heavy object it takes more energy to move it/ get it to move
     
  14. Dec 21, 2014 #13

    sophiecentaur

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    This is not accurate. As I wrote before, the same momentum is transferred to the gun (same as the bullet butt in the opposite direction) but for a massive gun, the speed is lower so the Kinetic Energy is even less. A gun of infinite mass would not recoil at all and it would take no energy. If that is counter-intuitive, I can sympathise but there is no argument against such basics of Mechanics.
     
  15. Dec 21, 2014 #14

    A.T.

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  16. Dec 21, 2014 #15

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    Some regard it as energy of the gun/firearm, some as momentum of same, some as energy of gun/firearm + operator, or as momentum of same. In the first pair of definitions the gun/firearm is regarded as a second projectile whose energy or momentum must be absorbed by the shoulder of the shooter. Force, impulse, impact questions become the topic of endless debates.
     
  17. Dec 21, 2014 #16
    Oh ok Ya I see what your saying and you are right i was wrong.
     
  18. Dec 21, 2014 #17

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    The momentum is all from the bullet and gasses. All the rest is just a middle-man in conveying that momentum to you. Changing the weight of the mechanism in the gun will change the nature of the force you will have to resist. If the mechanism is heavier, it will come back more slowly than if the mechanism is lighter, but the total momentum you have to resist is the same. So the question is if it is better to have a sharp, light-weight recoil or a slower, heavier recoil. I don't know enough about shooting to add any more.
     
  19. Dec 21, 2014 #18

    sophiecentaur

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    Yes, the momentum is the same but that isn't what causes the discomfort. The Energy in that recoil will be what does the damage to you and a heavier gun (the bit that reacts against the bullet's exit momentum) will give you less energy to deal with. A very massive gun would be ideal - if it weren't for the fact that you couldn't lift it and hold it steady to aim or run with it. The anti recoil mechanisms just make life the least worst for the person holding the gun by spreading the momentum transfer over a longer time. But I know nothing about the details of the Physics of those systems. You can be sure there will be a great deal of Science BS talked about it by experienced users who may be excellent shots and 'know what they like'. It's such a subjective business that the 'correct' detailed Mechanics of the situation may not be very relevant, in fact.

    I have seen some videos of large field guns firing and they appear to buck about in an alarming fashion. For the sake of accuracy of aim, that must be prevented until the shell has left the barrel.
     
  20. Dec 21, 2014 #19

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    Good point. That sounds reasonable.
     
  21. Dec 21, 2014 #20
    For the purposes of this discussion, I didn't want to deal with the recoil caused by the bullet and gasses leaving the front of the barrel, I just want to focus on the felt recoil caused by the moving parts of the semi-automatic action. Those parts are just starting to move rearward as the bullet leaves the barrel and are still moving rearward relatively long after the exit of the bullet and gas out the barrel's front.

    So back on the AR15 forum, it got brought up that people who use low mass bolt/buffer systems have a tuned gas system. The gas block is an attachment about halfway down the barrel. There is a hole drilled into the barrel and the gas block clamps onto it, and the gas feeds from the barrel to the gas block through that hole and then travels down the gas tube back into the receiver where it directly impinges onto the bolt carrier. A normal gas block has a fixed sized hole for the gas to travel through. A tuned gas block has a screw that allows them to turn it ever so slightly one way or another to open and close the size of the gas hole. Somebody remarked that if you tune the gas system right, the lower mass system will have less recoil. But wouldn't it still stand true that at that same amount of gas tuning the heavier system would still have even less?

    This was the exchange:

     
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