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    Question Steel stress

    In which part of the chamber is the pressure/stress most high? Is it at the walls surrounding it or at the breech?

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    I believe that the pressure is the same throughout the chamber.

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    Another way to look at it is that pressure builds up to a peak and then decreases over milliseconds. Over that time the bullet moves forward, exposing more of the chamber and barrel to those higher pressures (further down). Based on that, I'd guess the highest pressure (even while evenly distributed, as Kurt suggests) to be felt well forward of the case? If designing for maximum hoop stress, that stress would likely be at whatever parts were exposed at peak pressure.

    Someone here surely knows.

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    The pressure is equal on every surface inside the chamber. Certain features or shapes may react differently to that pressure, but the pressure is the same.

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    Case mouth is different than midcase pressures. I will check the data at work tomorrow and see what the difference is.

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    Another problem is that the pressure generation is transient, more like shock loading. Some metal behave differently between slow and rapid loading. I would question if the pressures are uniform.

    The pressure peak is probably in the microsec range than millisec.

    Tom

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    Recollection is that many are around half a millisecond for the peak, decreasing after that.

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    A ballistician said that he was often given shotgun loads to test. He rejected most of them because they had used slow burning powder to keep pressure up over the length of the barrel and the thin wall down the tube could not handle it.

    Mil, do you know the speed of sound in the gases during the burn? I know that high temperature and pressure raise Mach 1. Does it form shock waves in places like the shoulders of rifle cartridges?

    Bill

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    Re peak time, a bullet traveling 1000 FPS would exit a 24" barrel in 2 milliseconds. Since this is from a dead start, if it accelerated linearly, it would take 4 ms. Since it accelerates faster at the beginning, it would be significantly less than 4. A high velocity rifle like a 220 Swift going 4000 FPS will take less than a millisecond to exit.

    In a handgun with a 6" barrel at 1000 FPS, we are also talking about less than a millisecond for the whole event.

    Of course, the peak will be shorter. There will be a period where pressure builds up before the bullet moves far. I have no idea what that is.

    Bill
    Last edited by 9100; 01-29-2020 at 07:53 AM.

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    Quote Originally Posted by 9100 View Post
    Re peak time, a bullet traveling 1000 FPS would exit a 24" barrel in 2 milliseconds. Since this is from a dead start, if it accelerated linearly, it would take 4 ms. Since it accelerates faster at the beginning, it would be significantly less than 4. A high velocity rifle like a 220 Swift going 4000 FPS will take less than a millisecond to exit.

    In a handgun with a 6" barrel at 1000 FPS, we are also talking about less than a millisecond for the whole event.

    Of course, the peak will be shorter. There will be a period where pressure builds up before the bullet moves far. I have no idea what that is.

    Bill
    I love this kind of thinking.

    At the exact moment in time just before the bullet moves moves but the powder is burning how high could pressures get? If that elapsed time is so infinitesimally small, could the case pressure greatly exceed the allowable hoops stress without deforming the chamber because the chamber hasn't had time to react? If so, at short durations how strong could something be?

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    Just checked QuickLoad and calculates the pressure and velocity at various place as the bullet moves down the barrel. With the printed out graph a person could get good estimates of both at various barrel lengths. Just grabbed a 222 printout and it shows 3626 PSI initial pressure, probably before the bullet starts to move. Bullet travel at Pmax (45,827PSI) is 1.35 inches.

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    Quote Originally Posted by FredC View Post
    Just checked QuickLoad and calculates the pressure and velocity at various place as the bullet moves down the barrel. With the printed out graph a person could get good estimates of both at various barrel lengths. Just grabbed a 222 printout and it shows 3626 PSI initial pressure, probably before the bullet starts to move. Bullet travel at Pmax (45,827PSI) is 1.35 inches.
    There is a limitation for the time a pulse takes to travel along a solid like steel. If you strike the end of a steel bar, there is a definite lag before it reaches the other end. Collins uses this effect in their mechanical filters in radios. They magnetostrictively pulse a bar, probably a nickel alloy, that had larger and smaller sections that resonate at different frequencies, and detect it at the other end magnetically.

    Mechanical filter - Wikipedia

    How that relates to firearms is another thing. I suspect that the pulse would have to be extremely short or the chamber wall very thick for it to have much effect.

    We see another effect every day in machining. Turning a part on a lathe, below a certain surface speed you get a rough surface and as you increase speed, it suddenly gets smoother, I think largely because you are no longer giving it time to deform so it cuts off clean.

    Roofing tar comes in a paper drum and you have to cut off pieces. Slicing some off with a knife is next to impossible because it sticks t o the blade. Strike it with an axe and it shatters because the wedging force exceeds the tensile strength of the tar.

    Similarly, I think a chamber subjected to excessive pressures building up gradually would stretch while the same pressure applied suddenly would cause it to shatter.

    Bill

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    Quote Originally Posted by TDegenhart View Post
    Another problem is that the pressure generation is transient, more like shock loading. Some metal behave differently between slow and rapid loading. I would question if the pressures are uniform.

    The pressure peak is probably in the microsec range than millisec.

    Tom
    Just looking at the Q/L printout for 222 and peak pressure is at approximately .31ms (1.35 inches) bullet exits the barrel (22inches) at .976ms with a pressure of 7062PSI. I hope no one quoted this post yet. Less than a millisecond to exit the barrel is probably right. Abbreviation for microsecs is different.

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    Our test results show that mid-case pressures average 3000psi to 5000psi higher than the case-mouth pressures. This is measured with a Kistler 6215 piezo electric Gage. The time under the pressure curve is in milliseconds. I can ask the electronics techs here for some number examples.

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    Here goes a limited knowledge guess while we are waiting for MilGunsith's actual test data. Three high pressure areas #1 center of the case body, reason center of the rapidly expanding gas #2 beginning of the shoulder, reason gas running into the shoulder before being choked into the neck #3 area just behind the bullet at PMax. In the case of my 222 example 1.35" down the barrel, reason high velocity gas running into the slow moving bullet. Again my limited knowledge would say the low pressure point would be the case neck, reason venture effect.

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    We have test ported a barrel every inch following the groove. Waiting for approval to release the data.

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    Quote Originally Posted by MilGunsmith View Post
    We have test ported a barrel every inch following the groove. Waiting for approval to release the data.
    Any luck? I'd like to see this if it's possible.

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    OK just checking out my knowledge from what I have read on this site and from following links posted on this site. I am not a gunsmith. First the pressure is the same on all inside walls of the cartridge. The amount of brass present in different areas of the case transfer different loads to the different parts of the chamber wall and the bolt face. The heavy brass at the base of the cartridge along with the grip of the cartridge to the chamber wall reduces the stress on the bolt face relative to the chamber walls. The defining factor for the chamber wall is the hoop stress calculation for the cartridge chamber pressure applied to the cartridge case and the chamber wall.

    This is my basic understanding of the situation so let me know where I have things wrong.

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    In fact chamber pressure is not in a steady state ,because the solids and gases are in motion at a very high velocity.Barrels and chambers of firearms are not amenable to thin shell analysis,but much more complex thick shell theory.The behavior of pressure waves and energy transformations has been widely researched with respect to larger bores since the discovery of destructive waves in guns by the French in 1884...You can conduct your own experiment by "ringing" the chamber of your favourite gun.

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    Quote Originally Posted by lanb View Post
    OK just checking out my knowledge from what I have read on this site and from following links posted on this site. I am not a gunsmith. First the pressure is the same on all inside walls of the cartridge. The amount of brass present in different areas of the case transfer different loads to the different parts of the chamber wall and the bolt face. The heavy brass at the base of the cartridge along with the grip of the cartridge to the chamber wall reduces the stress on the bolt face relative to the chamber walls. The defining factor for the chamber wall is the hoop stress calculation for the cartridge chamber pressure applied to the cartridge case and the chamber wall.

    This is my basic understanding of the situation so let me know where I have things wrong.
    In a static small chamber with nothing going on the pressure inside that would be the same every where. Inside the rifle cartridge during the ignition till the slug leaves the barrel, it is very dynamic with a rapidly changing volume. Even the first Microseconds of ignition are complex. The primer shoots a hot jet of "flame" through the center of the charge, powder shape and chemistry affect burning rate as the burn progresses through the charge, shock waves (as mentioned by john.k) bouncing around, directional movement of molecules and burning grains. Unbelievably complex stuff going on in there. Any measurement of pressure is just averaged over micro seconds and really for just one area of the expanding volume of the system. Milgunsmith may have revealed something that his employers do not want to be published so we may not learn the rest of what our inquiring minds want to know.
    If a person wanted a pretty good average approximation of pressures and time, one could use a program like QuickLoad to find pressures (again average estimated) by starting with a set caliber and powder charge then doing incremental changes of the barrel length. The program could tell you the pressure, percentage of powder burned, and I think the temperature of the charge for each barrel length. Any way Q/L gives you a lot more info than most of us need.
    9100 and TomD have both mentioned shock waves going through the steel in the barrel and chamber more complexity that is hard to even fathom. Seems that the safety factor that is needed is several times what you can estimate based on average pressure at any given time.


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