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Thread: Trying to understand ammo..

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    Regular Member self preservation's Avatar
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    Trying to understand ammo..

    I have been reading up on ammo specs and to say the least, I'm a little confused. I hope someone here has some insight into some of my stupid questions. I read where the 9mm has a bullet diameter of 9.01 (0.355 in) and has a maximum pressure of 35.001 psi (SAAMI standards). The .357 mag on the other hand only has a bullet size of 9.1 (.357 in) and a maximum pressure of 35.000 psi. So it would seem to me that for all intents and purposes, with the exception of the .357 being .2 inches larger and the 9mm having 1 extra PSI, they seem to be the round. Would anyone else agree with this or am I missing something?
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    You are missing things, like a feel for magnitudes of numbers.

    0.2 inches is almost a quarter of an inch (0.25"). Your pressures might be thousands of psi (35,000 psi, I'd write ksi or kpsi).

    One must carefully differentiate between nominal and actual dimensions, look to the engineering drawings of the bullets.

    All of the bullets mentioned, and pressures, are very similar, but the devil is in the details. ETA - read how SAAMI pressures are actually measured.
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    It's not just size and pressure generated. All sorts of stuff goes into describing the ballistics of any given round - and that changes as the length of the barrel (and no other variables) changes. See http://www.ballisticsbytheinch.com/ for data that will really confuse you.

    Get past chronograph data (BBTI) into performance - flat shooting vs "shoots like a rainbow", deformation of bullet in ballistic gel, differences in temporary and permanent wound cavities due to differences in bullet design, and a whole lot more.

    But before you go beyond chronographing a round, remember the variances in load: standard, +P, +P+, and wildcat "I would not set it off from closer than three counties away" loads.

    Reloading manuals can help understand the differences between similar-looking bullets, such as your example of 9mm vs .357Mag. Some manuals also give you a short history of the bullet/round - how/why it came to be and why it survived.

    Just be sure to drop breadcrumbs as you explore your way around ammo. It may help you get back again.

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    Regular Member self preservation's Avatar
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    I'm still reading and trying to learn, but some things have blown my mind. Such as when I read that the 9 is ballistically superior to the .38 special. A side by side comparison of the 2 rounds lead my to believe that the longer cartridge (.38) must be "more powerful" based on nothing more than the "bigger is better" theory. Of course this is not true. Wonder why the .38 has a longer cartridge it they ain't gonna pack as much boom into it as they do the shorter 9?
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    How much 'empty' space not taken by powder in a cartridge is incredibly important. See our discussions of set-back of a bullet into the cartridge due to repeated stripping from the magazine. Too much set-back can raise pressures to KABOOM!
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    Regular Member JustaShooter's Avatar
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    The difference in diameter is actually 0.002" - 2 thousandths of an inch, or about the thickness of a human hair. So it isn't the diameter that makes a .357 magnum more powerful than a 9mm.

    The pressure spec is the same, 35kpsi or 35,000 - but that doesn't tell the tale.

    In the 9mm that max pressure is reached quickly but drops off quickly as well since there isn't much case capacity for powder (and tends to use a fast burning powder for that reason). By contrast the .357 magnum has much larger case capacity (and uses a slower burning powder) so the max pressure (or at least near max) is reached more slowly but is sustained much longer, producing much higher velocity.

    Compare a 124gr bullet out of a 9mm at 1130fps to a 125gr bullet out of a .357 magnum at 1,500fps - and that's the difference in the two rounds: longer sustained pressure producing much higher velocity in the .357 magnum.
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    38 spl has a longer case because it was designed for black powder, which has more volume than smokeless powder and there can be no voids in the case. The 357 is longer so it will not fit in a gun designed for 38 spl. NOW powder used for 38 and 357 are sometimes slower burning powders which take up more volume in the case. Slower powder means higher velocity in full loads with lower pressure than fast burning powder used in ACP cases. 357 also usually uses a heavier bullet than the 9 mm. Keep in mind also that the amount of steel surrounding the case of the ACP cartridge is thicker than the thinnest part of most revolver cylinders, they can take more pressure. Except in the case of large frame revolvers. But factory loads are loaded for the weakest gun, not the strongest. It is a safety thang.
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    Regular Member JustaShooter's Avatar
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    Quote Originally Posted by WalkingWolf View Post
    But factory loads are loaded for the weakest gun, not the strongest. It is a safety thang.
    It really isn't a weakest gun vs strongest gun issue, factory loads are loaded to SAAMI spec, and (at least today) guns are manufactured to handle ammunition produced to SAAMI spec, so technically it's a "spec thang".
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    Regular Member WalkingWolf's Avatar
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    Quote Originally Posted by JustaShooter View Post
    It really isn't a weakest gun vs strongest gun issue, factory loads are loaded to SAAMI spec, and (at least today) guns are manufactured to handle ammunition produced to SAAMI spec, so technically it's a "spec thang".
    Modern factory ammo IS loaded to be safe in older guns, rounds such as 32-20, 45-70, 30-06 specs are designed for wait for it~~~Safety. Sheeesh!
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    It is not the brass case that cannot take the pressure, it is the firearm. I take .38spl cases, remove metal and shorten them, and load to over 357 pressures for an obsolete rifle from 1905. The gun requires such pressure and velocity to function the self-loading action. It is not the case size but the firearm that is the determining factor.
    About the only example of modern factory ammo that I can think of, loaded for 2 different pressure levels, is for the 45/70. You can purchase ammo for trapdoor style actions (weaker) or modern power levels.
    It makes no sense for a business to sell, for example, .38 spl ammo suitable for Colt revolvers with heat treated cylinders, and a lower pressure type for pre heat treatment revolvers. Joe shooter just wants to know that if he buys .38 special ammo, it will go bang in his gun without blowing up. SAAMI specs were lowered in the past to accommodate weaker firearms and keep the lawyers happy.
    I have a different criteria for handgun ammo for myself and family. Reloadable so that we can practice a lot. Are you able to shoot fast and accurately with it. Son likes a 9mm with 17 shot magazine. Daughter shoots a S&W m19 357 mag but with very light handloads because she hates recoil. Wife likes a little Sig in .380 auto, I think just because it is 'cute', but she does shoot it well. I use an old .38 colt because it is light and comfortable and I like old guns.
    All the above do the job of a handgun, punch a hole in the target. If you want temporary wound channel, massive expansion and manstopping power, carry a rifle.

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    Quote Originally Posted by hovercat View Post
    It is not the brass case that cannot take the pressure, it is the firearm. I take .38spl cases, remove metal and shorten them, and load to over 357 pressures for an obsolete rifle from 1905. The gun requires such pressure and velocity to function the self-loading action. It is not the case size but the firearm that is the determining factor.
    About the only example of modern factory ammo that I can think of, loaded for 2 different pressure levels, is for the 45/70. You can purchase ammo for trapdoor style actions (weaker) or modern power levels.
    It makes no sense for a business to sell, for example, .38 spl ammo suitable for Colt revolvers with heat treated cylinders, and a lower pressure type for pre heat treatment revolvers. Joe shooter just wants to know that if he buys .38 special ammo, it will go bang in his gun without blowing up. SAAMI specs were lowered in the past to accommodate weaker firearms and keep the lawyers happy.
    I have a different criteria for handgun ammo for myself and family. Reloadable so that we can practice a lot. Are you able to shoot fast and accurately with it. Son likes a 9mm with 17 shot magazine. Daughter shoots a S&W m19 357 mag but with very light handloads because she hates recoil. Wife likes a little Sig in .380 auto, I think just because it is 'cute', but she does shoot it well. I use an old .38 colt because it is light and comfortable and I like old guns.
    All the above do the job of a handgun, punch a hole in the target. If you want temporary wound channel, massive expansion and manstopping power, carry a rifle.
    cat is correct .. shoot what you can accurately ... if that means a 380 or 22 so be it. I'd rather shoot a 22 lr and be a deadeye than a 50 cal and not be able to hit the broadside of a barn.

    A rifle is always a best choice if possible. Someone breaking into my house? I'm grabbing a rifle, not a handgun.

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