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combustion airsoft pistol

Built a miniature cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: KineticAmbitions » Tue Sep 29, 2009 7:14 pm

No the mass of the weapon itself determines how the kick feels, whether it accelerates or jerks. However the power of the kick is all determinant on the time it takes to recoil. Either way the same amount of energy is being absorbed.


Muzzle energy is NOT equal to recoil energy. The recoil energy varies based on the launcher's mass because recoil stems from conservation of MOMENTUM. For a given muzzle energy, recoil decreases as:
1. The launcher's mass increases
2. The projectile mass decreases

Try to visualize the results of the following:
You are standing in the middle of a road while a car accelerates toward you. On one side of this car, a 10 pound sledgehammer is secured to the frame so that the head protrudes a foot or so out the window.
When the car reaches 100mph, it passes by you in such a way that the sledgehammer strikes you square on the shoulder.
Now imagine firing a .30-06 rifle. Which would you rather do?
If recoil energy was equivalent to muzzle energy, both choices would have the same result.


Back on the real topic, that's an... interesting launcher. Have you tried to measure muzzle speeds yet?
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Unread postAuthor: blackhawk13 » Tue Sep 29, 2009 7:24 pm

i have no way to chrono it. i estimate 350-400 depending on the fuel/air mix. if you want an exact velocity i would gladly welcome you sending me a chrono :D
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Unread postAuthor: Insomniac » Wed Sep 30, 2009 1:02 am

KineticAmbitions wrote:
No the mass of the weapon itself determines how the kick feels, whether it accelerates or jerks. However the power of the kick is all determinant on the time it takes to recoil. Either way the same amount of energy is being absorbed.


Muzzle energy is NOT equal to recoil energy. The recoil energy varies based on the launcher's mass because recoil stems from conservation of MOMENTUM. For a given muzzle energy, recoil decreases as:
1. The launcher's mass increases
2. The projectile mass decreases

Try to visualize the results of the following:
You are standing in the middle of a road while a car accelerates toward you. On one side of this car, a 10 pound sledgehammer is secured to the frame so that the head protrudes a foot or so out the window.
When the car reaches 100mph, it passes by you in such a way that the sledgehammer strikes you square on the shoulder.
Now imagine firing a .30-06 rifle. Which would you rather do?
If recoil energy was equivalent to muzzle energy, both choices would have the same result.


Back on the real topic, that's an... interesting launcher. Have you tried to measure muzzle speeds yet?


Unless I'm severly mistaken, this is incorrect. Muzzle energy and recoil energy have to be equal, it's a fundamental law of physics. For every action there is an equal and opposite reaction. How the recoil feels is completely dependent on how quickly the energy is transferred to your body and how it is absorbed.

The example you gave isn't quite accurate, as when you fire a gun you never actually let it accellerate and then deliver it's recoil energy in one hit. You have the stock jammed into your shoulder so that you steadily absorb the energy as it is transferred to the weapon.

Picture yourself firing a 12 guage shotgun correctly, and then repeating the shot but this time with the stock about half a foot in front of your shoulder, while loosely holding the gun. Same energy in both scenarios, but it will be delivered all at once in the second shot and would probably do some damage.
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 30, 2009 1:27 am

blackhawk13 wrote:i have no way to chrono it. i estimate 350-400 depending on the fuel/air mix. if you want an exact velocity i would gladly welcome you sending me a chrono :D


Or you can use a couple of coke cans, this is for 0.20g BBs
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Unread postAuthor: blackhawk13 » Wed Sep 30, 2009 5:52 am

thanks. as soon as i get some more .20s ill try that and post the results
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Unread postAuthor: KineticAmbitions » Wed Sep 30, 2009 4:56 pm

Unless I'm severely mistaken, this is incorrect. Muzzle energy and recoil energy have to be equal, it's a fundamental law of physics. For every action there is an equal and opposite reaction. How the recoil feels is completely dependent on how quickly the energy is transferred to your body and how it is absorbed.

The example you gave isn't quite accurate, as when you fire a gun you never actually let it accellerate and then deliver it's recoil energy in one hit. You have the stock jammed into your shoulder so that you steadily absorb the energy as it is transferred to the weapon.

Picture yourself firing a 12 guage shotgun correctly, and then repeating the shot but this time with the stock about half a foot in front of your shoulder, while loosely holding the gun. Same energy in both scenarios, but it will be delivered all at once in the second shot and would probably do some damage.


You are severely mistaken. For every action, there IS an equal and opposing reaction. This is the law of conservation of momentum. Energy and momentum are always conserved, but it is much easier to see where the momentum went.

In the case of energy, you start off with a chemical potential energy in the powder and zero kinetic energy. In the case of a .30-06, this would be in the neighbourhood of 20,000 joules. Roughly one third of this is transferred to the projectile, another third is held by the propellant gases (in the form of kinetic energy, as the gas is heated and thus has a high particle speed, as well as having a net velocity - out the end of the barrel), and the rest goes to heating the barrel, with a small amount being transferred to kinetic energy of the firearm itself - the recoil energy. In a hunting rifle, recoil energy would make up less than one percent of the original chemical energy. Remember that energy is a scalar quantity, while momentum is a vector, and that the cartridge contains energy, not momentum. The fact that energy is scalar means that you can't "cancel it out". The same is not true of momentum.

Now we move to momentum. It must also be conserved. We have zero momentum to begin with, and that will continue to be the case until we add some. The bullet leaves the barrel with something like 15kg*m/s of forward momentum. To cancel it out, in the simplest case (no muzzle break) we need the same amount in the opposite direction. If our firearm weighs 10kg, it will be accelerated to 1.5m/s in a direction opposing the one of the bullet leaving the muzzle. We now have zero momentum, and physics is happy. If a 90kg man was holding the gun in such a way that he absorbed the recoil, his torso would be accelerated backward to roughly 0.2m/s, something that is perfectly manageable and shouldn't result in falling.

Now, imagine if energy was conserved. I do agree that my sledgehammer analogy was inaccurate, but a more accurate one wouldn't be a whole lot more favourable to the shooter. Assuming perfect stance, the 90 kg man's torso would be accelerated to about 11m/s. If it didn't kill him, it would certainly ruin his day. Kind of like being tackled by a very large man running as fast as an olympic sprinter, but the entire impact would be concentrated on a few square inches of your shoulder.
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Unread postAuthor: Ragnarok » Wed Sep 30, 2009 5:57 pm

As KineticAmbitions has already said, it is momentum that is equal in both the projectile (and propellant gas) and the firearm. Not energy.

Momentum is force multiplied by time, and is a vector quantity. Kinetic energy is force times displacement, and is a scalar quantity.

Think it through logically. Two things exerting a force on one another have to exert an equal (and opposite) force for an equal time - anything else is preposterous.
Conversely, the two object do not have to be displaced equal distances in each direction during the time they are exerting said forces - a situation that is preposterous in all but one circumstance (two objects of equal mass with a starting velocity of 0).

Anyway, I can very much assure you that KA is correct.

From here, knowing the recoil momentum (or impulse) means you can work out the firearm's recoil velocity, and from that, the recoil energy (via mv<sup>2</sup>/2), which is what determines how hard the kick is.
It's the energy in the kick that tells you how "hard" it is, but the momentum of the kick that is equal to the momentum of the projectile (plus the momentum of propellant gas leaving the muzzle, of course).

Anyway, recoil energy, disregarding propellant gas momentum, and assuming recoil velocity to be negligible compared to muzzle velocity can be figured out as simply as follows: M<sub>Bullet</sub><sup>2</sup>V<sub>Bullet</sub><sup>2</sup>/2*M<sub>Gun</sub>

KineticAmbitions wrote:You are standing in the middle of a road while a car accelerates toward you. On one side of this car, a 10 pound sledgehammer is secured to the frame so that the head protrudes a foot or so out the window.
When the car reaches 100mph, it passes by you in such a way that the sledgehammer strikes you square on the shoulder.

Ah, happy days.
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Unread postAuthor: blackhawk13 » Wed Sep 30, 2009 6:11 pm

this gun is now running on butane!!!!!!!! thanks to my friend for the syringe to measure it(even though the kid whose locker is next to mine thought it was for drugs) it is shooting better than ever. i tried the can test and it shot the can 10 feet! it went through both sides but not the bottom. i was using .12bbs. i will try again when i have .20s. i am using about 15 ml of butane gas. the problem is when the syringe fills with liquid butane :shock: i am trying to post new pics and a video. ill send the link when i do
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Unread postAuthor: blackhawk13 » Wed Sep 30, 2009 6:32 pm

here is the video

[url]<object><param></param><param></param><param></param><embed></embed></object>[/url]

and the link for the pics

http://www.facebook.com/album.php?aid=2019550&id=1117755651&l=6097c886b5
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Unread postAuthor: blackhawk13 » Wed Sep 30, 2009 6:34 pm

sorry the video didnt work. here is the link

http://www.youtube.com/watch?v=-gBvho5eHsw
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Unread postAuthor: Technician1002 » Wed Sep 30, 2009 7:41 pm

blackhawk13 wrote: the problem is when the syringe fills with liquid butane


Point the can up instead of down to prevent this. :D
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Unread postAuthor: blackhawk13 » Wed Sep 30, 2009 7:47 pm

Technician1002 wrote:
blackhawk13 wrote: the problem is when the syringe fills with liquid butane


Point the can up instead of down to prevent this. :D


i do but sometimes it still does a little bit. is butane bad to get all over yourself?
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Unread postAuthor: spudtyrrant » Wed Sep 30, 2009 8:03 pm

KineticAmbitions wrote: Energy and momentum are always conserved
not according to Steven Hawking http://en.wikipedia.org/wiki/Black_hole ... on_paradox :D
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 01, 2009 12:14 am

blackhawk13 wrote:I do but sometimes it still does a little bit. is butane bad to get all over yourself?


:shock: Yes, butane is known to cause infertility in pubescent males :shock:

Just kidding, it's fine ;)

If you stand the can properly, put the syringe on the nozzle with the plunger full closed, then press gently so the rising butane pressure pushes the plunger out, you should have no spillage problems.
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Unread postAuthor: Insomniac » Thu Oct 01, 2009 1:10 am

Ah, thanks for clearing that up. I'm starting to understand it now. So would that mean that two objects with the same momentum won't neccissarily have the same amount of kinetic energy?
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