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Pressure question

Discussion in 'The Ammo & Reloading Forum' started by jdon72, Feb 28, 2010.

  1. jdon72

    jdon72 New Member

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    According to SAAMI the average max pressure for 45acp is 21,000. If you have a powder and the max load is bringing the pressure to 17,000, are they saying that any more powder in that load is just being blown out the barrel and not much more velocity can be obtained? 5 inch barrel. What do you guys think and know.

    THanks

    J
  2. armedandsafe

    armedandsafe Guest

    There is truth that an overload, but safe load can blow powder out the barrel before it has all burned to develop pressure. This is particularly true in the snubbies.

    A lot has to do with the burn characteristics of the powder. A fast powder will build pressure faster. A slow burning powder will develop pressure more slowly and be more apt to expel the bullet before the powder has developed full pressure.

    Look to your reloading manuals and also check the powder manufacturer's web sites to determine the best load for YOUR gun, paying particular attention to the barrel length they use in their testing. Most manufacturers will answer questions through their web sites, also.

    So, the answer to the question you asked is, "Yes, no and maybe." :D If a manufacturer or reloading manual says a particular load will build 17,000 PSI crush at maximum load, they are just saying that a heavier load is not recommended. There might be safety reasons, or it might be that the case will not hold more with that particular bullet and still let the bullet be seated to meet maximum overall length, or.... The thing is, they don't actually tell you why that is the maximum, but the rule of (keeping your) thumb is to not exceed what the book says is max.

    Pops
  3. The_Rifleman

    The_Rifleman New Member

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    They are saying if you go any further, do it of your own accord not theirs.

    I know some people that just load their cartridges to the highest charge it will take. They take great pride in obtaining velocities of a 30-06 in their 308.

    I do not hold with that recipe for disaster. I stay mid-range with most of my cartridges; only with my 357 did I need a charge close to the suggested max load to obtain acceptable accuracy. And that is what marksmanship is about, accuracy.

    As far as "more powder in that load is just being blown out the barrel." A faster burning powder, no. A slower burning powder, yes.

    But you really need to stick with the powders the book suggest, and if you exceed the recommended max charge, do it with GREAT caution and KNOW what pressure signs to look for. As soon as you see pressure signs BACK OFF.

    I don't personally see a need in achieving velocities above my guns safe capabilities, and/or reaching maximum pressure. If I need that kind of velocity, I'll buy a gun capable of achieving those velocities safely.
  4. Bindernut

    Bindernut Well-Known Member

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    Another thing to be aware of concerning pressures listed for max loads.

    In the situation you've got posted, SAAMI spec is 21k but that max load load you've got listed shows 17k.
    Either the case won't hold any more powder to push the bullet any harder, the next "step" up the charge scale will result in a load that is over 21k, or possibly the next step is inconsistent in pressure so they can't give any solid results on how it performed in the barrel they used to test.

    And the obligatory disclaimer too...
    Just because a manual has a load listed as max for the gun they used doesn't mean it is max (or above or below) in your gun. Your gun might reach max pressure before you hit that charge or it might be mild with that charge. That's why every manual will tell ya to start at the minimum and work up to the max, but they'll never tell you that if the pressure signs look mild that you could exceed their posted data. It's a CYA thing to keep the lawyers off their backs and their customers safe. :)
  5. Bindernut

    Bindernut Well-Known Member

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    Now that's the best way I've ever heard that put Pops! I like that "keeping your" addition!
  6. jdon72

    jdon72 New Member

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    Thanks for the input guys...I have carbine that has a 16" barrel and I have seen some info for loads on the carbine, but not much info. Most is on pistol...that being more popular. I just like to get the more velocity for longer ranges with the carbine. Obviously, I do not want to use the fastest burning powder...I have more room in the barrel to burn the powder.

    J
  7. Bindernut

    Bindernut Well-Known Member

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    Yup, if you're loading for a carbine you can better utilize a slower burning powder. It's gotta be hard to find carbine-specific load data for most pistol cartridges.
    Can't help ya with that one.
  8. jdon72

    jdon72 New Member

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    Thanks binder.

    J
  9. Popgunner

    Popgunner New Member

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    To your first question, if the manual is listing a max load that's only 17k of pressure in the 45acp that has a max of 21k I would think they stopped adding powder when the gun cycled well & wasn't tearing brass apart. Pressure over time is what determines velocity & how violent the slide will action. Heavier bullets will accentuate the slide force due to longer barrel time. Pistol powder is "degressive" burning meaning it starts fast, hits high pressure & burns out quick.

    Speaking of pressure in rifle barrels, rifle powder is "Progressive" burning meaning the surface deterent, shape & size of the grains & composition of rifle powder means it starts out slow & adds pressure longer. Even so, the 1906 50k psi 30-06 service cartridge in a 24" barrel builds 45k of pressure in the first 1/2 inch of bullet travel, & is up to 50k max pressure at only 1" of bullet travel. At 3" of bullet travel this load is down to 30k pressure. At 14" of bullet travel pressure is down to 10k psi. When the bullet leaves the barrel pressure is 7.5k psi. Expansion ratio of this cartridge in a 24" barrel is 7.4, meaning the case volume times 7.4 is equal to the volume of the barrel the bullet travels in. The same7.4 expansion ratio in the .22LR equates to a revolver shooting a bullet from a cylinder & out a 2" barrel. The .22LR bullet will actually slow down inside any barrel over 14" long. Just runs out of pressure compared to barrel friction.

    Anyway there are several reasons a "max" load listed may be lower than the SAMMI max including gun functioning & plain lead bullet loads that won't stand more pressure/velocity without leading. Also muzzle flash.

    If you want the max velocity in your carbine try some of the new "Leverrevolution" ammo with duplex powder loads. Zoomier than handloads.

    .308 ammo can very well have higher velocity than 30-06 loads if bullets are 150 grains or lighter. The max pressure is 50k on '06 & now .308 presure max is 62k. (was 52k till recently).
  10. jdon72

    jdon72 New Member

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    Interesting...very informative. Thanks pop
  11. The_Rifleman

    The_Rifleman New Member

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    I'm talking about cartridges loaded with 208 a-maxs grossly overcharged getting pressure signs out the wazoo just to prove their 308 is as good as a 30-06. They practically need to hammer their bolt open but trudge right along endangering themselves and everyone around them.
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