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Posted: 3/16/2013 3:46:24 AM EDT
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Barrel lengths being equal, is the gas prouced In a carbine length gas system more than a mid length because of the duration of time from when the bullet passes the gas port and when it leaves the barrel which would allow more time for gas to be forced into the port?
If so then it would be correct to say that the shorter the barrel length for any given system the less gas it would have? |
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Barrel lengths being equal, is the gas prouced In a carbine length gas system more than a mid length because of the duration of time from when the bullet passes the gas port and when it leaves the barrel which would allow more time for gas to be forced into the port? If so then it would be correct to say that the shorter the barrel length for any given system the less gas it would have? Don't assume all gas ports are drilled the same size. |
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Barrel lengths being equal, is the gas prouced In a carbine length gas system more than a mid length because of the duration of time from when the bullet passes the gas port and when it leaves the barrel which would allow more time for gas to be forced into the port? If so then it would be correct to say that the shorter the barrel length for any given system the less gas it would have? Don't assume all gas ports are drilled the same size. I'm not. I'm saying if the setup were to remain constant and the only variables are to be location of gas port and length of barrel. |
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Keep in mind that Eugene Stoner's original conception of a direct gas impingement system with ideal length was shortened at the request of uncle sam to allow a bayonet to be mounted. This is bullshit. The first carbine gas systems all had bayonet lugs removed, and threads that started almost immediately after the FSB. The first production carbine length gas system able to mount a bayonet was the R651, which was a commercial/export model, and not developed for or used by the military. ~Augee |
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Look up "dwell time AR-15". This. The gas produced is the same in the same length barrel. The amount of time the gas system is pressurized varies with the length of the gas system. Thanks. Looked this up and it gave me the info that I was asking about. |
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When the rifle is fired the chamber pressure is roughly 55,000psi. As the projectile moves forward down the barrel this gas is given more volume (the barrel) to expand into so the pressure goes down. The amount of pressure in the barrel when the projectile passes the gas port is the pressure that is delivered to the BCG to cycle the action. So this means the closer the gas port is to the the chamber the higher the pressure will be when the projectile passes the gas port. With the gas port farther away, the gas has more volume to expand into prior to passing the gas port reducing the pressure. The amount of time the gas is available to cycle the action, a.k.a dwell time, is a result of the distance between the gas port and the muzzle (simplified). When the projectile passes the gas port the gas system is pressurized, once the projectile exits the muzzle pressure drops essentially to zero. |
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Keep in mind that Eugene Stoner's original conception of a direct gas impingement system with ideal length was shortened at the request of uncle sam to allow a bayonet to be mounted. This is bullshit. The first carbine gas systems all had bayonet lugs removed, and threads that started almost immediately after the FSB. The first production carbine length gas system able to mount a bayonet was the R651, which was a commercial/export model, and not developed for or used by the military. ~Augee I didn't say anything about carbines. ETA- I can see how I wasn't very clear. I was talking about the length of gas tube being shortened therefore lengething the amount of barrel between the gas block and muzzle, not a reduction in barrel length. |
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When the rifle is fired the chamber pressure is roughly 55,000psi. As the projectile moves forward down the barrel this gas is given more volume (the barrel) to expand into so the pressure goes down. The amount of pressure in the barrel when the projectile passes the gas port is the pressure that is delivered to the BCG to cycle the action. So this means the closer the gas port is to the the chamber the higher the pressure will be when the projectile passes the gas port. With the gas port farther away, the gas has more volume to expand into prior to passing the gas port reducing the pressure. The amount of time the gas is available to cycle the action, a.k.a dwell time, is a result of the distance between the gas port and the muzzle (simplified). When the projectile passes the gas port the gas system is pressurized, once the projectile exits the muzzle pressure drops essentially to zero. That's not exactly accurate. The peak pressure is about 55k psi, but that peak is not reached before the bullet leaves the chamber. The peak pressure is not reached until the bullet has traveled part way down the bore. How far it has traveled before reaching that peak depends on propellant burn rate and bullet weight. |
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I found this on WIkipedia, I know, I know but it still is a good graph to help you understand the basics of how the pressure, velocity, and bullet travel work.
Scroll half way down the page till you see the graphs. Internal Ballistics |
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Desert and Gamma have linked the same post... I actually came across that post while looking up dwell time and it really gave me a clear, more than basic understanding of the system. So according to this while the closer proximity of the port may mean greater pressure initially entering the port, the dwell time seems to be the major factor in the equation. |
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Desert and Gamma have linked the same post... I actually came across that post while looking up dwell time and it really gave me a clear, more than basic understanding of the system. So according to this while the closer proximity of the port may mean greater pressure initially entering the port, the dwell time seems to be the major factor in the equation. Yes. The period of time between ignition of the powder and the bullet passing the gas port, and then the amount of time after that before the bullet leaves the bore are the two key factors, and the latter is pretty much not a factor for any but the shortest of barrels, |
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Quoted: Quoted: When the rifle is fired the chamber pressure is roughly 55,000psi. As the projectile moves forward down the barrel this gas is given more volume (the barrel) to expand into so the pressure goes down. The amount of pressure in the barrel when the projectile passes the gas port is the pressure that is delivered to the BCG to cycle the action. So this means the closer the gas port is to the the chamber the higher the pressure will be when the projectile passes the gas port. With the gas port farther away, the gas has more volume to expand into prior to passing the gas port reducing the pressure. The amount of time the gas is available to cycle the action, a.k.a dwell time, is a result of the distance between the gas port and the muzzle (simplified). When the projectile passes the gas port the gas system is pressurized, once the projectile exits the muzzle pressure drops essentially to zero. That's not exactly accurate. The peak pressure is about 55k psi, but that peak is not reached before the bullet leaves the chamber. The peak pressure is not reached until the bullet has traveled part way down the bore. How far it has traveled before reaching that peak depends on propellant burn rate and bullet weight. Yea, I kinda simplified it for comprehension purpose. |
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Desert and Gamma have linked the same post... I actually came across that post while looking up dwell time and it really gave me a clear, more than basic understanding of the system. So according to this while the closer proximity of the port may mean greater pressure initially entering the port, the dwell time seems to be the major factor in the equation. Oops 
Bigbore from ADCO's post with the cutaway photos also is good reading to help get an idea of how the system works. Mechanically it's a very simple system but the simplicity of the parts belies the complexities of all the forces at work - Stoner was really a genius, or else very lucky (and most great inventions are a combination of the two  )
There are multiple variables at work, not just dwell. Timing (my term) is set by how far the gas port is away from the chamber, this is when the energy pulse is delivered to the BCG. The location of the gas port also determines the gas pressure that will be charging the system. The combination of pressure, dwell (how long) and gas port size (gas volume restrictor) determine how much energy is delivered to the BCG. As you can see, the gas port location for timing and pressure, and the dwell, are bound together, as changes in one directly changes the others - longer dwell equals faster timing and higher pressure, shorter dwell equals later timing and lower pressure. You could as an example make a 16" barrel using any gas system length from pistol to rifle, and the system could be made to operate. By picking a reasonable and more "correct" gas system length though, you get an overall system that is more reliable, has a larger operating envelope (ie, can accept more variation in operating parameters and maintain function), and has less stress and wear on parts by being a more efficient system with fewer extraneous forces at work. Quoted:
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Keep in mind that Eugene Stoner's original conception of a direct gas impingement system with ideal length was shortened at the request of uncle sam to allow a bayonet to be mounted. This is bullshit. The first carbine gas systems all had bayonet lugs removed, and threads that started almost immediately after the FSB. The first production carbine length gas system able to mount a bayonet was the R651, which was a commercial/export model, and not developed for or used by the military. Yes, Wirebrush is a little mixed up. The original shorter versions were made for various military applications, but didn't involve a bayonet nor were they bayonet compatible. The aircrew survival rifle prototypes, the XM177 types etc were the original shorter versions, the 14.5" barrel with bayonet compatibility was later as you mention. It's worth noting that Stoner was not involved in design work for the shortened versions, that was all after Colt had acquired the AR15. To my knowledge the only shorter gas system that Stoner was party to designing was the KAC length, developed in the (IIRC)(early?) 90s and intended for a 16" barrel 7.62x51 rifle. As far as bayonet though, it would be more correct to say that the barrel was lengthened while using the same (carbine) gas system, in part to have compatibility with a bayonet. One of the differences is that with shorter barrels and gas systems, the pressures are much higher and things are happening faster, so small changes have much greater impact than with longer barrels. |
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