1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Strange physics question... (gun related, but kinda long)

Discussion in 'General Gun Discussions' started by Dionysusigma, May 9, 2004.

  1. Dionysusigma

    Dionysusigma Well-Known Member

    A.K.A. Dumb questions in kinetic energy transfer

    As I was relaxing this afternoon, I was playing Deus Ex and good 'ol Doom 2 :D , and a thought came to me (at first, I thought it was just a headache, but with pictures).


    So anyway, there are two main types of energy, potential and kinetic, and both can exist in different forms. Potential energy can be anything from a mass held at a height, chemically stored, and so forth. Kinetic is light, motion, heat (particles in motion), etc.

    Reading different threads on this board, I found that pistol calibers coming out of barrels that were longer than their original intended use (i. e., a 9mm from a 20" barrel) may actually be hindered by that added length by slowing them down. Likewise, larger calibers coming out of a much shorter barrel (7.62x54R out of an M44) can leave a lot of the powder unburned.

    Kinetic energy can transfer from object to object, sometimes changing its form: Car tires (kinetic energy is their rotation) spinning on a concrete surface (friction) causes heat (another form of kinetic energy).

    Also, the Heisenberg Uncertainty principle may apply as well--using a photon (light) to determine a particle's (friction, in a sense) location and/or state will invariably change that particle's state... example: You like to play pool in the dark, and have a glow-in-the-dark cue ball. The racked balls (which are not glow-in-the-dark) are the mass you are measuring with the cue ball. After the break, you know where the cue ball made contact with the racked set, but you have no idea where the numbered balls are now. The cue ball's energy (motion) is directly responsible for their change of state (motion, change of location, etc.)

    Tie this into a gun: energy is put into the bolt/slide/whatever when you, the operator, charges the gun (kinetic energy). That energy is transferred into the striker/hammer, and stored as potential energy. That energy is released (kinetic motion) when you pull the trigger, causing the hammer to fall on the firing pin (kinetic motion) and detonate the primer and eventually the powder (chemically stored potential energy now being released as heat).

    The gases inside the cartridge expand due to the heat, forcing the bullet out (kinetic heat/motion of expanding gases transferred to projectile). As the bullet moves down the barrel, the barrel gets hot (due to friction between the bullet and rifling, and heat from the gases). The bullet leaves the gun, followed by the expanding gases that propelled it, which results in (more often than not) muzzle flash.

    That's what gets me. If a 9mm is hampered by a long barrel to the point it slows down, shouldn't there be no muzzle flash? If the projectile is being slowed, that tells me that the burning powder/expanding gases aren't doing their job, so to speak. They aren't pushing the bullet, so they must be losing their energy. Why, then, does the flash still happen? For that matter, why is it still comparably loud?

    Someone please inform my ignorant self... why, oh why? :banghead: :( :confused:
  2. Preacherman

    Preacherman Well-Known Member

    Not necessarily. There may be some grains of propellant still burning - it's just that there aren't enough of them left to produce enough pressure to accelerate the bullet any more. These few grains still burning may produce a muzzle flash.
    The sound of the explosion is the same - it's got to come out somewhere! :D
    Heisenberg? Heisenberg? Didn't we ban him from THR a few months ago? :confused:

  3. 7.62FullMetalJacket

    7.62FullMetalJacket Well-Known Member

    You are assuming that the amount and type of propellant is a fixed issue. The 9mm cartridge is designed to propell the bullet based upon certain limiting factors. Since the 9mm is "usually" fired out of a short barrel, the powder is designed to burn very fast so that the bullet gets maximum acceleration. Let's asume the desin barrel is 4 inches.

    If you put that same pistol catridge in a carbine with a 20 inch barrel, you have not only a further bullet patch where it is being squeezed by the barrel (creating friction), but you have 5 TIMES the volume to fill. SInce the propellant is fast burning, most of the expansion occurs rapidly and the greatest acceleration occurs in the first so many inches of travel. After that, the volume of the chamber (barrel) is so large compared to design specifics that the small amount of gases are unable to create enough pressure to overcome the drag of the barrel on the bullet.

    As Preacherman stated, the noise is coming out regardless. The flash comes from the last few traces of burning propellant.

    SMGs use different ammunition that burns slower powder and builds much greater pressure to address your very problem.
  4. Bog

    Bog Well-Known Member

    Wonderful example of Logic being a way of Going Wrong with Confidence.

    I have to hand it to you, Dionysusigma, your logic is flawless - but oversimplified.

    The power in the cartidge burns at a specific rate. Sure, and fine, but the incandescence at the end of the barrel will be, as stated, the continuing combustion of that power, and indeed possibly barrel-oil burning, amongst other things (such as any wadding in the round, or anything) will contribute to muzzle-flash.

    The gasses in the barrel, producing the "bang" will not have fully expanded in the barrel - hence the sequence of baffles in a suppressor, to allow said gases to expand fully behing the bullet. You'd need a darn long conventional barrel to permit total gas expansion before the round left the muzzle. That comparable loudness of which you speak is a litre off of gas coming out of a 10 centilitre vessel, and shoving a bunch of air our of the way in doing so. Sound is just compressed air, after alll.

    Even then, if it's a faster-than-600-odd-miles-an-hour-round, it'll go "crack!" due to it's breaking the sound barrier.

    Very good reasoning, Dion, all the same.
  5. Standing Wolf

    Standing Wolf Member in memoriam

    Heisenberg may have felt uncertain, but the Warren Commission sure didn't.
  6. Oleg Volk

    Oleg Volk Moderator Emeritus

    Report will happen but, because of lower pressue at the muzzle, will be less severe. Compare NAA Mini Mag with 1 1/8" barrel to a rifle with an 18" barrel.

    Muzzle flash will be reduced but probably still visible in low light. Steel-jacketed ammo might also throw sparks.
  7. molonlabe

    molonlabe Well-Known Member

    Heisenberg Uncertainty principle does not apply at a macroscopic level.
    Last edited: May 9, 2004
  8. Archie

    Archie Well-Known Member

    Physics... remarkable study....

    Muzzle flash is the external burning of whatever powder is left over. (And anything else combustable in the barrel, as Bog mentioned, but I'm trying to stay simple.) Some powder is always left over, from a three inch barrel or a thirty inch barrel. However, I am passing sure that the longer barrel will give less flash than the shorter. One would need either special camera equipment or light meters that I don't have to test this proposition.

    Muzzle blast or report is caused by the shock wave generated by the pressure excaping from the barrel. 9x19 operates at a (peak) pressure of 35000 psi. Even in a 20 inch barrel, the pressure at moment of bullet release is pretty high, compared to the 14.7 psi of regular (sea level) atmospheric pressure. A soda pop can makes a "pop" when opened, and that pressure level is pretty low (comparated to a firearm).

    I keep hearing the theory about rounds "slowing down" due to a longer barrel. The US Army manual for submachines shows a higher velocity from the 45 ACP M3A1 submachinegun than from the M1911A1 pistol. The M3A1 has a 10 inch barrel (someone correct me if I'm wrong, I'm not looking this up...) and the M1911A1 has a five inch barrel. So the round seems to be more efficient in the longer barrel.

    Now let's go extreme (everyone put on a lab coat and a mad scientist monocle): We have a 30 foot barrel. All the powder in a 9x19 cartridge is going to either burn or refuse to burn. Boyle's Law shows the pressure at the muzzle of the 30 foot barrel is going to be 462 psi. (Based on a pressure of 35,000 psi at the end of a 4" barrel.) I'm pretty sure the bullet would leave the barrel, but velocity is going to be lowered.
    By the way, for a 16" barrel, the residue pressure is 10,400 psi, and a 20" has about 8,400 psi left. Velocity is not singularly determined on pressure, as the amount of time being accelerated is also a factor. The 16 and 20 inch barrels both push for a longer period of time, but I don't know the formula to determine velocity from this information.

    Now what I don't know: I've never done a test to see how much energy is required to drive a bullet through a barrel. Does the "longer push time" mentioned in the preceding paragraph overcome the fricton factor?

    I don't have either a 45 ACP or 9x19 carbine to compare. I do have three .44-40s, a 3", a 4 5/8' and a 16" carbine. Perhaps I can get some information from those.

    Anyone in the Los Angeles area with a pistol caliber carbine want to do some research? Let me know.

    Oh. Submachinegun ammo. The US never made pistol ammo specifically for submachineguns. Nor is it likely any major power issued two guns in the same caliber that required separate ammo, like 9x19 pistols and submachineguns. It just does not make sense. Please do not include the 9mm Glisenti... that's another sad story altogether.

    Feel free to remove your labcoats.
  9. mete

    mete Well-Known Member

    Scientific studies of shooting have shown that after 10 seconds your ability to hold a gun steady decrease and becomes worse the longer the time. I think Heisenberg Principle describes that perfectly !!...Besides Heisenberg is one of the very few who had the smarts and courage to write a one page doctoral thesis !
  10. P95Carry

    P95Carry Moderator Emeritus

    With the not inconsiderable ''Mu'' - friction factor .. even if the initial acceleration is very rapid ... there still needs to be considerable pressure left in the barrel just as bullet emerges ... but lower than would still contribute to further acceleration. ''Velocity maintainance'' if you will.

    There MUST always, be a pressure release if bullet makes it out the muzzle .. even if 4 feet long .... if it exits thru the results of powder burn and thus gas pressure, then .... has to be some residual left ... just may be much less ''bang'' ... and flash limited to completion, or not, of the propellant burn.

    In 9mm .... a high pressure round .... fastish powders will almost usually have ceased active combustion and been exhausted ..... expanding gas is all that's left. Minimal flame but some bang!
  11. Arc Angel

    Arc Angel Well-Known Member

    :D I can't believe I actually read this entire thread! Will somebody, please, tell me why I did this? By any chance could it be that, 'Heisenberg thing' - again. ;)
  12. Archie

    Archie Well-Known Member


    Tell me you're kidding. Please.
  13. molonlabe

    molonlabe Well-Known Member

    Heisenberg might have posted here.

    or he might not have.
  14. Dionysusigma

    Dionysusigma Well-Known Member

    So, if there was such a barrel that allowed total gas expansion, the bullet would get stuck every time, right? All right, this makes sense now. :)

    And, in the same theoretical universe, one could make a barrel several yards long that, when used with ammo loaded to exact specifications, could reach near total gas expansion only to have the bullet come out the end going a measly 1 fps. Report would still be there, although it'd be no louder than a bottle of champagne opening.

    And the pressure of the air outside the barrel compared to the gas pressure inside would make a difference too, I suppose. If fired in a vacuum, what then?
  15. caseydog

    caseydog Well-Known Member

    And as to the sound of things , unless the bullet were to really slow down or was designed to be subsonic, you're goin to have a "sonic boom".Ray
  16. Oleg Volk

    Oleg Volk Moderator Emeritus

    Another reason for using polygonal rifling in SMG barrels...reduced friction.
  17. Standing Wolf

    Standing Wolf Member in memoriam

    Thank you! That explains everything. I feel so much better now!

    Truth to tell: I utterly loathed, detested, and despised story problems all the way through high school and my sole semester of college mathematics. Trains going east versus automobiles going west, quarts turning into milliliters, electron spins and mass: all of it made me give up in despair. I understood the Heisenberg principle well enough to pass a physics test in high school, but all my knowledge vanished into thin air before I even started college.
  18. Arc Angel

    Arc Angel Well-Known Member

    :) Wow! I had no idea you guys were, all, so bright. I'll bet you could answer, almost, anything. OK, then:

    (1) 'Which came first, the chicken or the egg?'

    (2) 'Why did the chicken cross the road?'

    (3) 'Was, 'Fuzzy Wuzzy' really fuzzy?'

    (4) 'How many angels can fit on the head of a pin?'

    (5) 'If 19 pigeons are sitting on a fence, 4 hop down onto the ground, and I shoot 2 of them, how many are left, actually, sitting on the fence?'

    (6) 'Two rabbits are confined inside the same cage; one of them is, 'in heat'. Given a normal gestation period of 65 days and a typical birth rate of 8 to 10 bunnies per litter, with 2/3's of them female, how many bunnies will there be at the end of the year?'

    NOTE: An envelope containing the correct answers has been locked inside a vault guarded by Arthur Anderson & Associates. (The former accounting firm for Enron.) The first person to get ALL six questions right, wins a free THR ball cap. The clock is running, guys, so ... . :D
  19. P95Carry

    P95Carry Moderator Emeritus

    Arc Angel ..... it really is time you got that prescription changed!!! :D :neener:
  20. Mal H

    Mal H Administrator

    1) The egg. The final gene mutation (evolutionary type of mutation) that became what we know today as a chicken had to occur in an egg.

    2) To quote Albert Einstein - "Did the chicken really cross the road, or did the road move beneath the chicken?" In other words, how do we know the chicken actually crossed the road? Without that essential fact, we cannot truthfully answer the question posed.

    3) Yes, Fuzzy Wuzzy really was fuzzy. Just 2 weeks ago, I found an old Fuzzy Wuzzy childrens book at a flea market in PA and was surprised to find that FW was printed with fuzzy stuff all over him.

    4) None. All past accounts of angels have them human sized and unipositional, i.e., one cannot occupy the same space as another.

    5) None. There are two reasons why: a) pigeons don't "sit" on fences, they stand on them; b) any remaining pigeons on the fence would have been frightened away by the gunshot.

    6) Undetermined. It is not known if the other original rabbit was a male or female. Therefore there might be any number from 0 (the original bunnies could have died during the year) up to the maximum number allowed by your given facts which would be achieved if there were 10 bunnies in each litter and each female was immediately "with bunny" after the shortest time period possible for them to mate after their birth and all their offspring lived to maturity. Several of those factors were not included in the question.

Share This Page