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  1. #1
    Marksman

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    Flat based FMJ VS concave/recessed 9mm bullets for reloading

    Was wondering if there are any difference when using a flat based FMJ bullet vs a concave/recessed bottom bullet to reload with? Will be reloading on a Hornady LnL progressive press.

    Thanks,
    Art.

  2. #2
    Sharpshooter

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    .

  3. #3
    Grandmaster gregkl's Avatar

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    It would change the pressure but if you are using a loading manual with the proper bullet, the manual should adjust the charge accordingly.

    Bullet weight, type and seating depth all contribute to the final outcome.
    Easy choices, hard life.
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  4. #4
    Marksman 55fairlane's Avatar

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    Supposedly, both are designed to shoot the same, but in some cases one will shoot better,this is due to the flat base having more bearing surface, I know in rifles for instance, my 22-250 likes a flare case over a boat tail.....you will have to load some of both and see if one gives you better groups

  5. #5
    Grandmaster

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    Besides aiding in seating, I've never understood the whole boat tail concept.
    It would appear a flat base, with a larger bearing surface, would better catch the expanding gasses.
    NRA Instructor/Dormant U.S.Marine/ NRA Benefactor-Life

  6. #6
    Grandmaster Bigtanker's Avatar

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    Quote Originally Posted by Mgderf View Post
    Besides aiding in seating, I've never understood the whole boat tail concept.
    It would appear a flat base, with a larger bearing surface, would better catch the expanding gasses.
    The boat tail is for aerodynamics. It closes the air behind the bullet for a cleaner flight. That technology is used in all over the place. Have you seen these on the back of semi's? Boat tails. It really helps close the air in neatly behind the trailer, thus making it more aerodynamic, thus giving the truck better fuel economy.



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  7. #7
    Plinker

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    My experience loading 9mm: flat based JHP (Precision Delta) are slightly more accurate than concave base FMJ (Everglades). But, not significant enough for my purposes to care. I believe the flat bases are a more consistent Bullet design and even distribution of pressure. Concave bases have more bearing surface but are susceptible to the lead melting/ vaporizing.

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