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Thread: Tube gun from scratch

  1. #21
    Gunco Rookie puddle pirate's Avatar
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    Ok, thanks again. Uhm, possibly a foolish question tho. Wouldnt this be a risk on the rifles that are of the semi auto nature also? Just wondering.

  2. #22
    gunco irregular moleman's Avatar
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    Yes I agree with the kernel that there are safer designs and that its not deeded in semi auto. On an ar15 the recessed bolt face isn't an issue because the bolt has to close and turn before the cartridge can be fired. If you jammed the firing pin forward on it you would have a bad day when the gun fired before it locked.

  3. #23
    Gunco Veteran muttman's Avatar
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    not ment to put a kink in this post but, what about the sr41, floting fireing pin and feed lips. last issue of SGN home build stated feed lips are still on the bolt face. the best and safest way is to contact the BATF and ask ( and get a letter) if you feel the need to keep the lips.
    the best reason I have found for the lips is the fixed firing pin is pushing the case away from the bolt face and the lower lips help keep the case "stuck" to the bolt face.
    muttman

  4. #24
    Gunco Veteran Markp's Avatar
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    Quote Originally Posted by puddle pirate View Post
    And also the reason this matters is I did promise my instructor to make sure as best I could that the design would be in compliance with applicable state and federal laws. So this why I keep asking questions.
    Well if you were an 07/02, a tube gun in full auto would be in compliance. However, I understand the point that he probably doesn't want you designing a machine gun.

    I do however appreciate the discussion since I plan on building some SA builds and I feel it's important to know both how the FA action and SA action work so as to avoid building something that is illegal. By understanding the FA action you can avoid making something that could function in a similar manner, and obviously understanding the SA action is critical to build an SA gun.

    Mark

  5. #25
    Gunco Veteran Markp's Avatar
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    Quote Originally Posted by puddle pirate View Post
    Ok, thanks again. Uhm, possibly a foolish question tho. Wouldnt this be a risk on the rifles that are of the semi auto nature also? Just wondering.
    I believe this is a possible issue on SKS's full of cosmoline.

    Mark

  6. #26
    No Hope For Me Coils's Avatar
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    not ment to put a kink in this post but, what about the sr41, floting fireing pin and feed lips.
    The SR41 used a spring loaded firing pin to prevent slam fire, as for the feed lips they are shaved down but not completely removed. They claim everything about the SR41 is ATF approved, but the feed lip thing confuses me.

  7. #27
    Gunco Rookie puddle pirate's Avatar
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    There is a neat little book I bought BEFORE I knew I was gonna do this project called " Submachine Gun Designers Handbook" It covers the Sten Mk2, PPSh 41" It's by desert publications. by DELTA PRESS. Good book that does a breakdown of the trigger groups and principles needed to be covered in designing a subgun.

  8. #28
    Gunco Veteran muttman's Avatar
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    Quote Originally Posted by Coils View Post
    The SR41 used a spring loaded firing pin to prevent slam fire, as for the feed lips they are shaved down but not completely removed. They claim everything about the SR41 is ATF approved, but the feed lip thing confuses me.
    that is the reason I posted here. I have never layed eyes on the letter from the ATF about feed lips, remove or keep. I was told (on a difrent form, sten build) that if I removed the feed lips then the gun would not work in semi auto. the only reason for the feed lips is to hold the case with a fixed firing pin. that is the main reason I started a 9mmAK and not a sten. there is a post here that uses a AK bolt welded to the carriar making it a non rotating bolt that still uses a floting firing pin in blow back with no problems, and other home builds (sten, pps43, pps42) that keep the feed lips but have a spring on the firing pin, and I agree ther is a better (safer) way but the leving the feed lips makes sence to me for now. if some one find's a ATF letter on feed lips please post it. Im sure the ATF gives a long hard look at kit subguns just for the ease of putting in a FA bolt. muttman

  9. #29
    gunco irregular moleman's Avatar
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    I shamelessly coppied this from Tommers atf letter on weaponeer about the bolt. #3 in the atf response states the feed lips should be removed.

    This refers to your correspondence dated April 9, 2007, to the Firearms Technology Branch (FTB), Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), which accompanied your submitted Suomi bolt. In your letter, you request guidance regarding this bolt, which you have designed for a semiautomatic version of a Suomi M31 machinegun. Specifically, you want to ensure that (1) the bolt would be acceptable for use in a closed-bolt semiautomatic version of a Suomi and (2) the modified bolt would now be considered a U.S.-made part for purposes of 92 2(r).

    The FTB examination of your bolt design revealed that it has been fabricated from a standard Suomi machinegun bolt with the following alterations or changes:

    1. The overall diameter of the bolt has been reduced by approximately .045.
    2. The feed lip has been eliminated from the bolt face.
    3. A large slot has been milled into the center section of the bolt body, presumably this slot will allow a hammer to strike the firing pin.
    4. The fixed firing pin has been eliminated and the bolt modified to allow for the installation of a separate spring loaded firing pin.

    We find that your bolt has been sufficiently modified from its original machinegun fixed firing pin configuration and meets all of our guidelines on modifications for use in a semiautomatic closed-bolt version of a machinegun.

    Although the bolt has been extensively modified at its core, it is still a bolt body of Finnish manufacture and for purposes of 922(r) would still be considered as a Finnish-made (imported) part.

    Once again, we strongly recommend that you submit your semiautomatic Suomi M31 to FIB for a determination as soon as you have completed it. Without actually examining a sample of the manufactured firearm, our Branch is unable to make a formal classification of this weapon. After the evaluation, we would provide you with a classification letter.

    The FTB examination of the barrel retaining system (or mounting block), barrel, and barrel shroud that you have sent us reveals that your proposed weapon will have a newly manufactured barrel which is approximately 17 inches in length.

    Also, your newly manufactured system (mounting block) has been redesigned to preclude the mounting of a standard Suomi M31 machinegun barrel.

    A weapon incorporating your newly designed barrel retaining system (mounting block) and a barrel with a length of 16 inches or more would not meet the definition of an SBR and would not be a “firearm” as defined in 26 U.S.C. 5845(a)(3).

    With respect to designing your semiautomatic version of the Suomi M31 machinegun, we advise you to consider incorporating the following features and specifications:

    1. The receiver should be of substantially smaller diameter than a standard Suomi sub-machinegun receiver to preclude the use of an unmodified Suomi machinegun bolt.

    2. A steel rail should be installed in the receiver to prevent use of fully automatic bolts. The rail should be installed in the receiver by a secure, full-fusion seam weld.

    3. The feed lips should be eliminated from the bolt.

    4. An open-bolt firing mechanism with a fixed firing pin must not be used.

    5. A hammer or striker mechanism incorporating a positive semiautomatic disconnector should be used.

    6. The sear mechanism and opening should be relocated or modified so that a standard Suomi sear mechanism cannot be introduced into the weapon.

    7. The firing mechanism should be designed to use an inertial striker (firing pin).

  10. #30
    Gunco Veteran muttman's Avatar
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    thanks molman, that is better than what I have found on the web. muttman

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