Barrel pressure ahead of the bullet

Discussion in 'ELR, Ballistics & Bullets Board' started by LC Tikka, Jul 9, 2017.

  1. LC Tikka

    LC Tikka

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    What are the varying pressures created in a barrel as the bullet pushes out the air. Does someone have the knowledge to calculate or give an educated answer to this Q.
    7mm barrel, 26" long, bullet terminal velocity 2850fps at sea level.
    Thanks LC
     
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  2. hogpatrol

    hogpatrol Gold $$ Contributor

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    My educated guess: Atmospheric pressure. There's nothing at the end of the barrel to change the pressure.
     
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  3. Ggmac

    Ggmac

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    Agree , atmosphere or close to it . It would mean the muzzle vel of the air would be almost the vel of the bullet , increasing in speed as the bullet moves down the barrel . Assuming no gas leakage .
    I was part a a group trying to shoot in a vacuum . I wasn't the brains part , just the guy making parts and keeping it safe from a weapons perspective .
    29.? In was possible but not practical ( 20+ yrs ago ) . All the interior pressure data of failed vac test is not available .
     
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  4. M-61

    M-61 "Quis Separabit" Silver $$ Contributor

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    Not meant to hijack.
    Do the gases ever surpass the speed of the bullet? Is there a brief point as the bullet exits the muzzle that it would have the gases passing it?
     
  5. T-shooter

    T-shooter

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    I don't know how to calculate it but I would imagine that as the bullet starts to move it will begin to compress the air in the barrel to some degree even though the muzzle end is open. The air mass will have an initial resistance to movement and air is very compressible and the highest pressure would be right in front of the bullet.
     
  6. hogpatrol

    hogpatrol Gold $$ Contributor

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  7. M-61

    M-61 "Quis Separabit" Silver $$ Contributor

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    Thanks hogpatrol!! To me it appears the IS a moment in time when upon exiting the muzzle the bullet travels downwind.
     
  8. DaveTooley

    DaveTooley Silver $$ Contributor

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    Yes the gas passes the bullet. Exit velocity is between 7000 and 9000 Fps. In a matter of a few inches the gas slows and the bullet exits the gas cloud.
     
  9. Sniper338

    Sniper338 Silver $$ Contributor

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    Yes. by law of physics it has to... watch videos on it..
     
  10. wholman

    wholman

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    was that unburned powder in front of bullet
     
  11. steve_podleski

    steve_podleski

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    When the bullet is moving supersonic, the air ahead of the shock does not 'feel' the bullet...so the pressure is near atmospheric.
     
  12. JasonT

    JasonT Silver $$ Contributor

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    But for a good portion of time, the bullet is subsonic in the barrel right?? Wouldn't it cause some sort of pressurization effect?
     
  13. damoncali

    damoncali Gold $$ Contributor

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    It's generally assumed to be atmospheric, although common sense tells us it will be slightly higher in reality. But comparing 14.7psi (atmospheric) with 55,000psi (chamber pressure) shows you why it's not really a concern.
     
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  14. jfseaman

    jfseaman Silver $$ Contributor

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    There is pressure ahead of the bullet from various factors.
    Not atmospheric pressure. The simplified Mythbusters hyper speed ping pong ball test shows it. Draw a vacuum and it goes faster. The ballistics speed men show there is a hard to break "wall" at about 4,700fps.
    Yes, absolutely. That us why I said "various factors" in commenting on the original post.

    If you move your hand through the air it compresses the air ahead of your hand. A fan for those hot days compresses the air ahead of the blades. Decompresses the air behind the blades. A bird flying through the sky compresses the air ahead of it's face, body and leading edges of it's wings.

    It's undeniable physics. Everything that moves though the air creates a pressure change.

    Back to can it be calculated? I'm sure there is a series of applicable formula to calculate it. I have no clue what they are. As I mentioned, the ballistics speed guys, the ones trying to go 1 mile a second. As in over 5280fps, seem to have hit a wall at about 4700fps except a couple examples. One reached 5178fps the other 5130fps.

    http://www.reloadersnest.com/detail.asp?CaliberID=100&LoadID=1147
    http://reloadersnest.com/detail.asp?CaliberID=22&LoadID=1200
    https://www.thehighroad.org/index.php?threads/mile-per-second-cartridge-is-it-possible.559484/
    https://www.shootersforum.com/wildcat-cartridges/79729-5280-fps.html
    https://thefiringline.com/forums/showthread.php?t=571430

    One mile per second is 3,600 miles per hour. The SR71 Blackbird is recorded as traveling at 2,700 miles per hour. The SR71 skin heats up from friction and compressed air.

    Yes absolutely the air compresses ahead of the bullet. Physics baby, physics.
     
  15. lpreddick

    lpreddick Gold $$ Contributor

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    I read something about this yrs ago. The study referred to a shadowgram, I believe...something like a real time xray...like flouroscopy. Photos were taking VERY fast and you could see what looked like a bubble coming off the muzzel (this was densely compressed air from barrel) then bullet exited barrel and pierced the edge of the "bubble" that immediately dissipated. Air in the barrel has to be pushed AHEAD of the bullet. Air compresses and exits muzzel at speed of bullet (mach 2-3+) The air bubble slows immediately and bullet pierces it with it's 2 shock waves.(tip and base). This study suggested this event may cause the bullet to slightly wobble, then return to stable flight. Benchresters refer to a bullet "going to sleep"...
     
  16. Texas10

    Texas10 Silver $$ Contributor

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    Every slo-mo video I've seen of a bullet being fired has shown burning powder and gasses exiting the barrel AHEAD of the bullet. Makes perfect sense to me as even when muzzle loading, you can't have a perfect seal against 50,000 psi using a soft metal like copper/lead.

    i have read of and spoken to shooters who load their bullets with a very large jump to the lands, and see very high velocities, with little or no "signs of pressure". Some say stuffing the bullet deeper in the case decreases case capacity leading to higher pressure, even when the signs are not there with resultant increase in velocity.

    I have a different theory; Imagine a bullet seated .180 short of the lands at the moment of ignition. Hot gasses and burning powder would be passing by the bullet as it makes it way to the throat and lands. At the moment it engraves fully in the lands there is, of course, an immediate rise in pressure behind the bullet, but something interesting happens ahead of the bullet. That supply of hot gas moving at thousands of feet per second down and out the barrel is suddenly cut off, and inertia takes over creating a vacuum in front of the bullet. But you can't have any thing more than a perfect vacuum, and that can't be adding much to the total velocity, so what else could be helping the bullet along?

    I suspect burning powder trapped between the bullet and the barrel as it engraves into the lands, creating a gas cushion, so the bullet isn't actually touching the barrel completely, so much as slipping along on a cushion of hot air (gas). Custom barrels with very fine finishes in the bore don't copper up like factory barrels. Perhaps because they sustain this cushion longer and better due to their smoother surfaces. Or the high pressure gas behind the bullet is replenishing the gas cushion as the bullet travels down the tube.

    Same theory used in Russian rocket propelled torpedos that vent part of the propellant gas in front of the torpedo creating a gas bubble for the torpedo to slip through. IIRC, they're getting 250 kts (that was what, 20 years ago) and feel the can go mach + underwater, or may have already done it.

    Or maybe I'm full of hot air....definitely full of gas:rolleyes:

    Anyway, just food for thought.
     
    Last edited: Jul 27, 2017
  17. lpreddick

    lpreddick Gold $$ Contributor

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    Interesting. Gases going past the bullet: even when a bullet is jammed into the lands.025 you can see a gap between bearing surface and grooves...try with a barrel stub. At moment of ignition and bullet release, some gasses go past bullet before it is fully engraved and bearing surface contacts groove. This is very hot gas and certainly can melt a little copper from bullet that is pushed ahead of bullet. Bullet seals and gases ahead of bullet slow a little then bullet meets gases and pushes it out. I have seen some factory barrels and a few custom barrels that have a copper deposit about 2/3 down the bore that comes out as a smudge on patch not linear streaks that suggest avulsion from bullet. Do barrels shooting only jumped bullets cooper foul more than jammed bullets? I jam only now and admit I have very little cooper fouling.
     
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  18. jfseaman

    jfseaman Silver $$ Contributor

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    Now we are getting some meat into this discussion.

    Yes at ignition gasses will slip past the bullet. Regardless of distance to the lands.
    Yes at 50,000 and up PSI gasses will slip past the bullet. Even at the modest sub 10,000 PSI of some calibers and pistols/revolvers, gasses get by.
    There in exists an interesting data point to examine. A 45ACP from a 1911 travels at ~860fps. A 45ACP from a revolver travels at about ~760fps. That's not that much of a loss considering that all the gasses are expelled once the bullet exits the cylinder and enters the barrel. There is no rifling in the cylinder. Yes, the slow motion videos show gasses existing the barrel ahead of the bullet in the revolver as well. Hmm. http://www.realguns.com/articles/728.htm

    I have been trying to figure out how to draw a vacuum in a barrel ala, MythBusters ping pong ball experiment, trying to exceed the 1 mile per second barrier (5280fps). Something for "free time", right now I am backed up with work so there is very little of that.
     
  19. Ballisticboy

    Ballisticboy

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    Two things here. First the original question on the gas pressure ahead of the bullet. Ignoring any gas leak past the bullet the maximum pressure ahead of the bullet to a first approximation will be what is known as the total pressure which is a combination of the static and dynamic pressures given by:-

    Total Pressure = Atmospheric pressure + 0.5 X V X V X Rho

    where Atmospheric pressure is 14.7 psi or 101325 pascals in English or metric
    and Rho is the air density which is 0.0023769 slugs per cubic foot in English units and 1.225 kg per metre cubed in metric.
    V is just the bullet speed in ft/sec or M/sec.
    The second point is the gas overtaking the bullet as it leaves the barrel. All projectiles, be they from hand guns, tank guns or artillery weapons, have gas flowing past them as they leave the barrel. It is known as the intermediate ballistic area and is distinct from both internal and external ballistics. It has been studied in great depth for many years as it has direct effects on accuracy and precision for all gun type weapons. It is a very complex area to study with no simple answers.
     
  20. CharlieNC

    CharlieNC

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    Ignoring the gas that leaks past the bullet.... Seems to me that the bullet moving down the barrel would induce behavior similar to a piston pump. We all are familiar with the pressure associated with pushing a piston in a closed cylinder, or a tight opening like pushing on a hypodermic needle. In this case pressure is proportional to Flow Rate(how much air is being pumped) x Air Viscosity x L(barrel length) / D^4 (barrel diameter). This factor could be determined if someone were motivated to do so.
     

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