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Thread: gas checks and velocity vs pressure

  1. #1

    gas checks and velocity vs pressure

    Hey,
    I've been reading about gas checks, and I'm a bit confused. From what I've read, it seems that the critical factor in using a gas check is velocity, and that the purpose of a gas check is to prevent gas cutting. Now, I don't understand why a higher velocity increases the risk of gas cutting, I would think that pressure would be the key ingredient (assuming the bullet is sized correctly), ie if it exceeds the yield strength of the lead and "pushes" its way around the bullet or something. If you are pushing a 9mm bullet at 1200 fps and 35,000 psi or so... why does that not require a gas check when pushing a rifle bullet to 2000 fps under the same max pressure does? With the gas check you still have mostly lead on the sides of the barrel, so it can't be the increased friction.

    Anyways, the point is... I don't understand why you need gas checks, or at least why they seem to be velocity dependent. Can someone please explain this to me?
    Last edited by MeestaSparkle; 01-31-2008 at 11:59 PM.

  2. #2
    Boolit Master
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    It's more pressure than velocity, but there's a lot of voodoo in cast bullets. I figger it is more important to know what they do than why they do it. A partial answer to your question would be the time factor. That bullet being pushed down the rifle barrel is going to be under pressure and heat longer and has more time to fail.
    Sometimes you gotta wonder if democracy is such a good idea.

  3. #3
    Boolit Master
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    Quote Originally Posted by MeestaSparkle View Post
    1. From what I've read, it seems that the critical factor in using a gas check is velocity, and that the purpose of a gas check is to prevent gas cutting.

    2. Now, I don't understand why a higher velocity increases the risk of gas cutting, I would think that pressure would be the key ingredient (assuming the bullet is sized correctly), ie if it exceeds the yield strength of the lead and "pushes" its way around the bullet or something.

    3. If you are pushing a 9mm bullet at 1200 fps and 35,000 psi or so... why does that not require a gas check when pushing a rifle bullet to 2000 fps under the same max pressure does? With the gas check you still have mostly lead on the sides of the barrel, so it can't be the increased friction.

    Anyways, the point is... I don't understand why you need gas checks, or at least why they seem to be velocity dependent. Can someone please explain this to me?

    MS,

    1. No. It's pressure. AND .... the amount of time that a bullet is subjected to that pressure.

    2. Gas cutting happens twice. How well you do your part determines if the cutting gas makes much difference or not. The two times are at first when pressure is coming up until the seal is established. We seem to recognize this and accept it. The second time happens at the muzzle.

    Gas is way lighter than a bullet. Once the bullet breaks confinement, gas rushes past. If it breaks uniformly, then the base cuts uniformly and stays in balance without yaw. If it breaks to one side first, then the jet force will kick the base the other direction causing yaw until the RPM can re-stabilize it. Even if the seal was broken evenly, if the base wasn't gas cut because of a check to protect it, then the bullet has a better chance for accuracy after the yaw settles down.

    The faster you are trying to push that cast bullet, the higher your muzzle pressure is going to be causing more inaccuracy.

    This is the reason for the old timers adage to use the longest barrel possible for cast along with start it off as gently as possible. Also why you hear to use the slowest twist rate that will stabilize is to minimize the pressure curve on the base. This is also why you hear to match the hardness to the pressure level. It really should be match hardness to the pressure CURVE.

    This is why cast bullets shoot "easiest" in the lowest velocity zone that they can be stabilized as muzzle pressure for rifles is the lowest to cause less yaw. It is also why handguns tend to perform better towards the top end of a cartridges capability. For handguns, it is most commonly observed by longer barrels being considered more accurate and easier to shoot.

    3. What is the case volume of a 9MM compared to your rifle cartridge? (speed of that pressure spike) Does that 9MM have a 22" barrel? Do you shoot the rifle for accuracy at the same range as the 9MM? Compare apples to apples and I think you will understand this which is answer 1. and 2. above. The big thing I haven't mentioned is the "plastic state" of lead under pressure. This takes time to occur which is why barrel time and cartridge volume (pressure curve) is the scientific answer to your question.
    Evaluate everything you read for safety and use common sense.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check