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My long range question = Wouldn't cylinder gap devour power on real long barrels?

Discussion in 'Handguns: Revolvers' started by ACES&8S, Mar 7, 2018.

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  1. Jack B.

    Jack B. Member

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  2. Varminterror

    Varminterror Member

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    The bullet traveling forward “disperses” the pressure. A bullet can stall in a sealed breech firearm without any vent, not just in a vented bore revolver. If the driving force (pressure x cross-sectional area) becomes equal to the frictive force of the bullet in the bore, it will stop (neglecting the slight extra “slip” to dispel the possessed momentum at the instant the forces balance).

    I recall in college loading down lower and lower charges of BP to replicate a legend I had heard as a kid - you could shoot one slow enough you could fire it straight up, never lose sight of the bullet, and catch it in your hand. I stalled a dozen bullets in a Navy replica doing so. Not enough push to clear the barrel... and yes, you CAN find a charge weight which just clears the muzzle, plus about 10ft, and then burn the hell out of your hand by catching it.

    A barrel can be long enough to stop a bullet. Just like a garden hose can be so long you can’t get water from it. But it doesn’t have to have a BC gap to stop a bullet, and no practical firearm will have a sufficiently long barrel to achieve it. A sealed breech would be ever so slightly pressurized when the bullet stops, which I assume would be vented around the cartridge when the brass springs back and breaks the seal against the chamber, but a revolver WOULD come to atmospheric pressure via the BC gap.

    It’s not magic folks...
     
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  3. Varminterror

    Varminterror Member

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    You are, however, absolutely wrong about the “cutting torch” part.

    The highest pressure, therefore highest escape velocity at the BC gap will happen the exact same regardless of barrel length - the max pressure happens typically with the bullet base slightly one side or the other of the BC gap.

    Flash will escape the BC gap as long as the bore is under pressure, aka the entire time the bullet is in the bore. If the bullet stalls, the flash through the gap will look exactly as it did if the bullet did not stall. There would not be some magical “reserve” of flame that suddenly realized it could not exit the muzzle, then ran backwards to rush out the BC gap. Study up on what pressure means, you’re way off base for actual science.
     
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  4. ACES&8S

    ACES&8S Member

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    I knew the answer, I just wondered if you knew it.
     
  5. Jim K

    Jim K Member.

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    The figures for the 18 inch test barrel and the 18.5" carbine barrel show far more discrepancy than can be accounted for by the normal barrel-cylinder gap. Either there is a real difference in ammo or in the guns that is not apparent in the figures, or there is a misprint somewhere. Normally, the revolver b-c gap will result in about 50 fps or less drop in velocity compared to a solid barrel.

    Jim
     
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  6. Ru4real

    Ru4real Member

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    Pressure is lost through a BC gap regardless of barrel length. But yes, more total pressure is lost in a longer barrel than a shorter one simply because a longer barrel provides more time to leak.

    How long does a particular barrel need to be so that a bullet is slower exiting than if the barrel was shortened? Cannot directly calculate. Depends on barrel, bullet, caliber and powder at a minimum and all of these are considered variables. For example, a bullet wears as it runs down the barrel. Longer barrels means more wear and thus friction and leaked gasses around the bullet are continually changing. Approximations can get the calculations within a few inches of actual barrel length, but that's about the best you can do. Consider that for the tiny 22 LR in a sealed breech rifle, most barrels longer than 26 to 30 inches begin losing velocity.

    The muzzle of a 30 inch 45 colt pistol barrel would be in the dirt in my holster.
     
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  7. Varminterror

    Varminterror Member

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    In a literal and physical sense, THERE IS NO LENGTH IN WHICH A REVOLVER BARREL CAN BE LONG ENOUGH TO FIRE SLOWER THAN A SHORTER BARREL DUE TO THE BC GAP LOSSES. Even a sealed breech firearm can be made with a barrel long enough to stall a bullet, the BC gap just helps that happen in a shorter length - but it is NOT wholly dependent upon the gap.

    All you really need to understand to be able to comprehend this:

    1) there must be positive pressure to be vented through the gap.

    2) positive pressure represents an accelerating force on the bullet.

    3) the longer a bullet is exposed to an accelerating force, the faster it will be traveling.

    This is high school physics stuff, fellas.

    We shoot these cartridges in 20” barrels all of the time. Even paltry little rounds like 38spcl, 32 short, 22LR... If the ~9% loss due to a 6thou BC gap were enough to cause a stall, then the 20” of barrel drag would be enough too. If you have any muzzle flash or black smoke you have additional potential for pressure (fuel to be burned), even if you didn’t have flash or smoke, you would then remain to have positive pressure for a lot of barrel inches. Only when the combustion is complete AND the pressure is fully expanded to just above a net balance with atmosphere, ONLY then will the bullet stall. BC gap or not.
     
    Last edited: Mar 9, 2018
  8. Ru4real

    Ru4real Member

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    This post is mostly true. But you only get a C on this high school physics test. Fix what is wrong in this post I quoted of yours and I will up your grade to a B.
     
  9. M118LR

    M118LR member

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    You just screwed in the barrel length of your choice on the Dan Wesson. It was the crane locking and tension on the end of the barrel that made the difference. JMHO.

    Pictures say a thousand words, once again JMHO.

    4 revolvers.jpg
     
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