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Thread: 5.56 vs 9mm penetration question

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    5.56 vs 9mm penetration question

    Is it really true that FMJ 9mm rounds actually penetrate and is likely to over-penetrate more than 5.56mm?

    I did a little research on this and that was what I was able to pulled up.

    I find that surprising because if 9mm can "penetrate" better than 5.56, then why is it that the 5.56 can penetrate body armor but 9mm can't?

    Or is there a difference between penetrating body armor and penetrating soft target and common housing material - brick, wood, siding etc..?

    Can some vet here clear this up?

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    Re: 5.56 vs 9mm penetration question

    Ever see the Mythbusters where they shoot under water? Rifle rounds disintegrate rapidly while pistol rounds stay intact.

    It's a velocity and mass issue. 5.56 and .223 move at something like three times the velocity of 9mm. But at the same time, they're like half the mass of a 9mm round.
    "I never learned from a man who agreed with me." Robert A. Heinlein

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    Penetration may depend on the medium the bullet passes through. I've lost track of the exact numbers, but my personal experiments shooting into stacks of well-soaked newspapers had the 5.56 going distinctly deeper than 9mm (solid rounds - moderate powder loads).

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    Against soft targets it's fairly simple, the velocity of a 9x19 bullet is much less than a rifle. The cross sectional area of a 9mm bullet is more than a 5.56 bullet. The thicker bullet is simply stronger due to more mass and is more resistant to breakage.

    Thus, a high velocity 'skinny' bullet , that is highly stressed due to its high velocity, will be more likely to come apart on soft target impact, than a low velocity, much less stressed, 'fat' bullet, assuming that both bullets are composed of the same materials.

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    Quote Originally Posted by Serious Account View Post
    Is it really true that FMJ 9mm rounds actually penetrate and is likely to over-penetrate more than 5.56mm?

    I did a little research on this and that was what I was able to pulled up.

    I find that surprising because if 9mm can "penetrate" better than 5.56, then why is it that the 5.56 can penetrate body armor but 9mm can't?

    Or is there a difference between penetrating body armor and penetrating soft target and common housing material - brick, wood, siding etc..?


    Can some vet here clear this up?
    The reason a 5.56 can defeat armor is high initial energy however that energy quickly dissipates hence why the round penetrates less in soft targets.
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    Additionally, soft BA is made from fibers. Rifle rounds are smaller in dia. with pointed tips...at high velocity. The tiny tip gets through the fibers easier. Kinda like handing a 10 year old a screwdriver vs. handing a bodybuilder an ice pick.

    That and triple the energy!
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    Quote Originally Posted by 200RNL View Post
    Against soft targets it's fairly simple, the velocity of a 9x19 bullet is much less than a rifle. The cross sectional area of a 9mm bullet is more than a 5.56 bullet. The thicker bullet is simply stronger due to more mass and is more resistant to breakage.

    Thus, a high velocity 'skinny' bullet , that is highly stressed due to its high velocity, will be more likely to come apart on soft target impact, than a low velocity, much less stressed, 'fat' bullet, assuming that both bullets are composed of the same materials.
    This. Shoot a 9mm FMJ into water or gel. Except for the rifling marks, chances are it looks unfired.

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    The 9mm is more likely to penetrate drywall better than the 5.56 because the 5.56 bullet destabilizes when it hits the first panel and tends to hit the second sideways. The 9mm bores straight and the drywall will fill the hollowpoint which kills expansion.

    In flesh, the 9mm defensive rounds penetrate roughly between 12 and 18 inches because that's the standard the FBI set. 5.56 rounds penetrate personal armor better because of velocity and bullet shape. 7.62x51 is even better because the bullets also pack greater mass.

    Boat tailed rifle bullets, if they don't mushroom, tend to destabilize when impacting a body. Non-expanding pistol bullets and round nosed rifle bullets tend to bore straight and penetrate deeply
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