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.300 blackout doesn't like 150gr. bullets

Discussion in 'Gunsmithing and Repairs' started by willymc, Aug 6, 2014.

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  1. willymc

    willymc Member

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    My brother sent me a .300 blackout upper that he picked up as part of a build. He wanted me to check it out while he was waiting for his lower. Eventually I was going to load some ammo for him. I took it to the range today, mounted on my S&W M&P 15 lower. It loved 125gr. Nosler green tips. It wouldn't function with 147gr. fmj or 150gr. fmj.. It would fire, eject, but not pick up another round and/or fire, not eject, and pick up another round and jam it. the 147's were factory and the 150's were my reloads. My .300 blackout upper eats both those rounds like candy with no problems. Any ideas why my brothers upper won't function with the heavier bullets.
     
  2. BBBBill

    BBBBill Member

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    Gas problem? Gas port undersized or partially blocked, leaking at the gas block, loose carrier key.....
     
  3. willymc

    willymc Member

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    Like I said, It shoots 125gr. Nosler green tips like a champ. Just curious what's up with the heavier bullets.
     
  4. rcmodel

    rcmodel Member in memoriam

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    What bbbbill just told you.

    The 125 grain loads work because they are using a different powder burn rate then the other loads and the gas system is sufficient to operate it.

    The other loads powder isn't.

    Do what he said and look for a loose gas key, miss-aligned or leaking gas block, kinked gas tube, missing bolt rings, etc.

    For whatever reason, the slower powder used in the heavier bullet loads is not getting to the bolt carrier & bolt.

    rc
     
  5. willymc

    willymc Member

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    Thanks Bill & RC. I'll give those things a look.
     
  6. winddummy

    winddummy Member

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    300 Black Out

    You may have to go to a heavier buffer to slow down the action. The heavier the bullet the more pressure buildup is needed for the BCG to move fully to the rear with time for it to pickup the round from the magazine. The 300 BLK Out is very picky about gas tube length and wheter it is a AR15 or M16 Bolt. A 5.56 round has a spread of about 24 gr in weight of bullets. A 300 has a spread of 110 grains.
     
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