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Thread: Carbine vs. Middy: Real world use

  1. #101
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    Quote Originally Posted by Eric D. View Post
    For clarification, is dwell the length of barrel before or after the gas port? Based on this quote I'm guessing its the latter.



    I was still goofed up thinking of the gas port as a regulator, which i now know its not. I convinced myself with this analogy: the instant before the bullet passes the gas port, the barrel is basically a fixed pressure vessel, like a compressor tank. If I punch a hole in a pressurized tank, air will escape at whatever pressure the tank is at, the size of the hole only determines how fast the tank bleeds off.
    The rifle fires, chamber pressure peaks, the case expands in the chamber, the bullet begins traveling down the bore -> chamber pressure continues to decrease as the bullet picks up speed continuing down the bore -> the bullet passes the gas port and combustion gasses start to flow down the gas tube ->

    This is the start of dwell time.

    pressure pushing back on the bolt increases until it is about equal to the pressure in the bore -> bolt unlocks and starts travelling to rear, hopefully at the same time the bullet leaves the muzzle (else you have FTE)

    This is the end of dwell time.

    brass case shrinks, bolt carrier continues continues rearward movement -> extraction

    Nothing is fixed, everything is changing with regard to time. You cant really think of anything as static except specific "moments".

  2. #102
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    Quote Originally Posted by Eric D. View Post
    I convinced myself with this analogy: the instant before the bullet passes the gas port, the barrel is basically a fixed pressure vessel, like a compressor tank. If I punch a hole in a pressurized tank, air will escape at whatever pressure the tank is at, the size of the hole only determines how fast the tank bleeds off.
    That's kind of an interesting way of looking at it. But you still have the dwell after the bullet passes the port as a factor.
    "You people have too much time on your hands." - scottryan

  3. #103
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    True, but breaking a system down into theoretical, instantaneous points, is often necessary for analysis - and this i where calculus comes in handy

    Quote Originally Posted by ZRH View Post
    Nothing is fixed, everything is changing with regard to time. You cant really think of anything as static except specific "moments".
    B.A.S. Mechanical Engineering Technology

  4. #104
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    Quote Originally Posted by Eric D. View Post
    True, but breaking a system down into theoretical, instantaneous points, is often necessary for analysis - and this i where calculus comes in handy
    And generally inconsequential and often way above the pay grade of the person posting.

    Most people seem to know almost enough to be dangerous.

    I understand that people are sitting in offices or home bored but it's been a he'll of a long time since I've seen a larger collection of inconsequential nonsense.

  5. #105
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    Quote Originally Posted by An Undocumented Worker View Post
    I guess I should have said; I'd like to see your logic behind that statement, because as far as I know gases/plasmas never reach the state of a nonflowing solid.

    Edit-Quoted post was restated while typing this reply

    I would certainly like to see the research behind his claim, cause that kinda stuff is quite intriguing though I still have trouble believing it.
    This is certainly possible and believeable. Do a little study into fluid dynamics. Here's another example that will blow your mind: On a daily basis, I deal with what is know as "Cavitation" in high pressure/temp steam lines. You've got 1050F superheated steam running @ 2500 + PSI through a pipe. When it reaches an obstruction in the pipe (such as a valve) it's flow is broken up, thus reducing the pressure. When this happens, the gas (steam) condenses rapidly forming water droplets, which then implode and erode the surface of the valve and pipe. So we coat the inside surfaces (sometimes made of inconel 718) with Stellite or other exotic, unique alloys to resist this erosion.

    Sorry, that was a bit of a rabbit trail, but the point is, fluids change state rapidly, and strangely when rapid changes in pressure, temperature, or flow path are experienced. I, for one, have always been curious to know what kind of erosion is experienced inside a gas block where it meets the gas tube. Just look @ the blast baffle of a suppressor, especially when used on an SBR, and the erosion it encounters. Add to that a 180deg shift in flow direction (from the flow path forward through the barrel, to reverse through the gas tube) that occurs inside the gas block. I'm suprised you don't see more gas block failures.
    Semper Paratus Certified AR15 Armorer

  6. #106
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    Quote Originally Posted by ZRH View Post
    The rifle fires, chamber pressure peaks, the case expands in the chamber, the bullet begins traveling down the bore -> chamber pressure continues to decrease as the bullet picks up speed continuing down the bore -> the bullet passes the gas port and combustion gasses start to flow down the gas tube ->

    This is the start of dwell time.

    pressure pushing back on the boltThrough the gas tube to the gas key, right? increases until it is about equal to the pressure in the bore -> bolt unlocks and starts travelling to rear, hopefully at the same time the bullet leaves the muzzle (else you have FTE)

    So essentially, back-pressure against the bolt lugs from the case cause it to stay locked. The lugs unlock when the pressure @ the key becomes higher than the residual pressure in the bore?

    This is the end of dwell time.

    brass case shrinks What causes this?, bolt carrier continues continues rearward movement -> extraction

    Nothing is fixed, everything is changing with regard to time. You cant really think of anything as static except specific "moments".
    Some questions above in bold...
    Semper Paratus Certified AR15 Armorer

  7. #107
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    The chamber behind the bolt's gas rings is essentially like a combustion chamber in an engine.

    The gas, however, is already ignited. It floods into the area behind the bolt actuating the rearward movement of the Carrier. The carrier moving rearward unlocks the bolt and excess gas is bled out the exhaust ports on the side of the carrier.

    This process happens regardless of bore/chamber pressure. That's why a short gas system extracts "harder" and chews up case rims worse than a rifle system. It's not an equalization of pressure process.... the bolt carrier is trying to pull the bolt out of the chamber regardless of pressure level.
    Last edited by markm; 04-01-11 at 11:37.
    "You people have too much time on your hands." - scottryan

  8. #108
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    Quote Originally Posted by ASH556 View Post
    This is certainly possible and believeable. Do a little study into fluid dynamics. Here's another example that will blow your mind: On a daily basis, I deal with what is know as "Cavitation" in high pressure/temp steam lines. You've got 1050F superheated steam running @ 2500 + PSI through a pipe. When it reaches an obstruction in the pipe (such as a valve) it's flow is broken up, thus reducing the pressure. When this happens, the gas (steam) condenses rapidly forming water droplets, which then implode and erode the surface of the valve and pipe. So we coat the inside surfaces (sometimes made of inconel 718) with Stellite or other exotic, unique alloys to resist this erosion.

    Sorry, that was a bit of a rabbit trail, but the point is, fluids change state rapidly, and strangely when rapid changes in pressure, temperature, or flow path are experienced. I, for one, have always been curious to know what kind of erosion is experienced inside a gas block where it meets the gas tube. Just look @ the blast baffle of a suppressor, especially when used on an SBR, and the erosion it encounters. Add to that a 180deg shift in flow direction (from the flow path forward through the barrel, to reverse through the gas tube) that occurs inside the gas block. I'm suprised you don't see more gas block failures.
    we have had pieces of equipment/ and steam lines fail due to cavitation and water hammer. Normally a quick sprint to a vent valve helps settle it down.

  9. #109
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    Quote Originally Posted by ASH556 View Post
    Some questions above in bold...
    Yes. Like what markm said about the carrier. Back pressure doesn't cause it to stay locked. It's minutiae really. [On a tangent there is a type of bolt that does use back pressure to stay locked, it's called a Blish lock and was used on early Thompson submachine guns]

    The brass case shrinks because of the sudden drop in pressure. If it tries to extract before the bullet leaves the muzzle you usually end up with FTE and a ripped case rim.

    @rob: Psh, calculus is easy. FEA on an entire system you don't have an accurate model of is the hard part.

    @entire thread: if you've read this far you might find this interesting http://handle.dtic.mil/100.2/AD880431 Sensitivity Study of Rifle Gas Systems
    Last edited by ZRH; 04-03-11 at 02:40.

  10. #110
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    Quote Originally Posted by ASH556 View Post
    This is certainly possible and believeable. Do a little study into fluid dynamics. Here's another example that will blow your mind: On a daily basis, I deal with what is know as "Cavitation" in high pressure/temp steam lines. You've got 1050F superheated steam running @ 2500 + PSI through a pipe. When it reaches an obstruction in the pipe (such as a valve) it's flow is broken up, thus reducing the pressure. When this happens, the gas (steam) condenses rapidly forming water droplets, which then implode and erode the surface of the valve and pipe. So we coat the inside surfaces (sometimes made of inconel 718) with Stellite or other exotic, unique alloys to resist this erosion.

    Sorry, that was a bit of a rabbit trail, but the point is, fluids change state rapidly, and strangely when rapid changes in pressure, temperature, or flow path are experienced. I, for one, have always been curious to know what kind of erosion is experienced inside a gas block where it meets the gas tube. Just look @ the blast baffle of a suppressor, especially when used on an SBR, and the erosion it encounters. Add to that a 180deg shift in flow direction (from the flow path forward through the barrel, to reverse through the gas tube) that occurs inside the gas block. I'm suprised you don't see more gas block failures.
    My understanding of Cavitation deals mainly with boat propellers and water tubines. What happens as the prop is moving through the water a low pressure area is created with the water being unable to flow fast enough to fill that low pressure area as a fluid. The water changes phase to a gas to fill that void and then the vaporous bubble collapses on it self creating a shockwave that erodes the prop/turbine.


    Erosion in a blast baffle of a compesated rifle is no different than the erosion of an exhaust valve of an automobile with a leaking exhaust manifold. Typically the exhaust is mostly free of O2, yet still has some unburned fuel in it. If there is a crack in the exhaust manifold near the head, as the pressure wave reverses, oxygen is sucked into the manifold and impinges against the exhaust valve, this O2 superheats the exhaust locally (due to the unburnt fuel in it) where it starts cutting the valve like a plasma torch. That is essentially what is happening to the blast baffle of a compesator/suppressor. Not cavitation.
    Last edited by An Undocumented Worker; 04-01-11 at 20:39.

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