open bolt vs closed bolt
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Thread: open bolt vs closed bolt

  1. #1
    Gunco Rookie jasbradley6's Avatar
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    Default open bolt vs closed bolt

    I'm an idiot when it comes to this stuff but I have a strong desire to learn. Could someone explain the difference between a open bolt vs a closed bolt design. I'm a visual person so a rough digram would help alot. I'm considering building a semi sub gun but before I start I want to make sure I thouroughly understand the legalities and other stuff.
    Thanks

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    Indian Admin Winn R's Avatar
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    The closed bolt trigger releases a hammer of some sort to strike a firing pin in a bolt closed on a breech.

    The open bolt trigger releases the bolt to go forward and strike the cartridge.
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


    "Never attribute to malice that which can be adequately explained by stupidity." Robert J. Hanlon

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    Chief Administrator 7.62x39's Avatar
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    In the simplest of terms, it's the position of the bolt when the trigger is pulled. All semi's (that I'm aware of) and some FAs are closed bolt. Meaning the bolt is closed and locked when the weapon is fired. Those that fire from a open bolt, strip, chamber and fire the round all in one motion.
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    Chief Administrator 7.62x39's Avatar
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    Damn speech recognition software. Winn beats out my two finger typing every time.
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    Indian Admin Winn R's Avatar
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    Quote Originally Posted by 7.62x39
    Damn speech recognition software. Winn beats out my two finger typing every time.
    Hey -- I'm doing it the hard way now!!! Sweat on my brow. (the sound card died)

    And you need to get south before you freeze up!
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


    "Never attribute to malice that which can be adequately explained by stupidity." Robert J. Hanlon

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    Chief Administrator 7.62x39's Avatar
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    Way ahead of you.
    Three weeks, I leave for San Juan from there it's on to Oranjestad Aruba then Willemstad Curacao, Philipsburg St. Maarten and Charlotte Amalie St Thomas. WooHoo.
    .

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    Gunco Regular Rikoshay's Avatar
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    I'm not fully aware, but I believe that open bolt designs have to be changed to a closed bolt to be a legal semi-auto. From all the other builds I've seen like the Soumi and M60 semi-auto's they are all converted to fire from a closed bolt with a hammer set-up.

    In the M60 design the operating rod holds the firing pin, and the bolt has a curved slot that the operating rod mates into. To assemble it into the receiver you have to twist the bolt while pulling to the rear (there is also a very stiff spring inside the bolt), then hold it with one hand and slide the bolt/op rod into the receiver tracks. It takes a lot of practice to do this and not have the bolt go flying through the air, due to that freaking spring.

    When you pull the trigger the whole assembly goes flying foward, picks up a round, the bolt stops, the curve in the bolt causes it to turn and lock as the op rod continues forward. Since the op rod holds the firing pin the pin also goes forward and strikes the primer and causes it to go bang.

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    Chief Administrator 7.62x39's Avatar
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    Quote Originally Posted by Rikoshay
    .... I believe that open bolt designs have to be changed to a closed bolt to be a legal semi-auto...
    That is correct.
    .

  9. #9
    Gunco Good ole boy kernelkrink's Avatar
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    In a pistol caliber firearm, the open bolt method has been used in both full auto and semi auto types. The bolt is made with a fixed firing pin in the extended, firing position. When the bolt completes it's forward travel after feeding a round into the chamber, the pin detonates the primer of the chambered round. The semi versions capture the bolt at the rearward position after each shot, requiring the trigger to be released before it resets for the next shot. Full autos only catch the bolt when the trigger is released.

    Prior to the early 80's several open bolt semi's existed, including the predecessor to the Tec9, the Demro "Tac" line, and a few Tommy gun lookalikes. One Tommy clone, the "spitfire" was declared an MG back in the 60s as it was designed to disable the semi function when one held the safety in a certain position when firing it. The others were/are still legal semi autos in unaltered form.

    In the 80's MAC/Cobray tried to get an open bolt MAC approved. ATF ruled that since ANY open bolt could be made full auto by simply removing or disabling the disconnector, ALL future semi autos had to be closed bolt. The older firearms are "grandfathered" as semis, but no new ones could be made.

    Not all closed bolt semis use hammer mechanisms, some use a striker like the UZI.

    To convert an open bolt to a closed bolt design you will have to remove the firing pin or "bump" from the original bolt and drill it for a retractable firing pin. The feed lip on the bottom of the breechface also needs machined away so it is flat with the breechface itself. The rest will be specific to your build type.

    Check over on Weaponeer, they have a lot of technical discussions on the various firearms and conversion types.

  10. #10
    Gunco Rookie jasbradley6's Avatar
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    thanks for the info, I understand now

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