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Thread: Tube gun from scratch

  1. #51
    Gunco Veteran Viper Dude's Avatar
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    Hello jlwilliams,
    Keep in mind that the Cobray and MAC design places the mag behind the trigger. A redesign would be needed for an AR-type lower trigger group.

    Also the Cobray/MAC bolts have become rather spendy and hard to find in recent years. A 7.62x25 mm version using a Czech M-24 mag seems like the thing to do.

    I believe that the square tubing used on these was an oddball size but no big deal if you can find a standard Cobray/MAC upper.

    VD

  2. #52
    Gunco Member jlwilliams's Avatar
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    The mag is behind the trigger, but the sear and hammer are behind the mag. There is a cubersome transfer bar arrangement in a MAC semi. You could easily enough make a traditional mag in front of the trigger gun. While they are not as cheap as they used to be, they do turn up on gunbroker, and things like firing pins and recoil springs would be easy to get. Another advantage would be that the telescoping bolt design would put the bolt over the mag well when closed with a few inches of the barrel inside the receiver. That would mean you would have less barrel sticking out front, so a legal 16" rifle would look shorter. Food for thought.

    I have a Cobray M11-9 bolt and recoil assembly. Maybe I should try a mock up.

    I like the CZ 7.62x25 idea.

  3. #53
    Gunco Veteran [486]'s Avatar
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    Quote Originally Posted by kernelkrink View Post
    Probably. Someone with a fancy calculater would have to do the math to get it right the first time, but shotgun shells are relatively low pressure rounds compared to say a centerfire rifle. SAAMI specs most shells around 11,000 psi up to 14,000psi for a magnum. That's low. The .308 specs at 62,000 and IIRC HK says the G3 bolt would have to be around 32lbs to work properly without the roller delay mechanism. Square inches of cartridgehead pressing against the bolt would factor in as well, 11,000 psi on a one inch head would equal 14,000 lbs pressing against it, a 2 inch one would double that to 22,000 lbs. Somewhere in there is the right answer!
    Orion's hammer did some calculations on a 9x19/7.62x25 bolts and used the bullets' energy as the recoil force, makes sense to me. The payload has the same bore diameter as the shell's head, and both sides have the same pressures exerted on them, so both will have the same force put to them, right?

    Anyways, so with the 12 let's assume you're gonna be throwing a 1 oz slug at 1,500 FPS or so, you want a bolt velocity of about 10-15 FPS Let's use 15 as it'll result in a lighter bolt, and easier calculations... You'll need a 100oz bolt to get the 15FPS bolt velocity, 100/16=6.25 So you'll need a 6 1/4 lb bolt in a closed bolt blowback shotgun firing slugs.

    Damn, all I needed was another project... Maybe even reduce the bolt weight by using a striker setup with a 1lb bolt and a 4lb firing striker to get some of the benefit of the whole open bolt inertial advantage that you guys were talking about earlier.

  4. #54
    Gunco Veteran Markp's Avatar
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    Quote Originally Posted by [486] View Post
    Orion's hammer did some calculations on a 9x19/7.62x25 bolts and used the bullets' energy as the recoil force, makes sense to me. The payload has the same bore diameter as the shell's head, and both sides have the same pressures exerted on them, so both will have the same force put to them, right?

    Anyways, so with the 12 let's assume you're gonna be throwing a 1 oz slug at 1,500 FPS or so, you want a bolt velocity of about 10-15 FPS Let's use 15 as it'll result in a lighter bolt, and easier calculations... You'll need a 100oz bolt to get the 15FPS bolt velocity, 100/16=6.25 So you'll need a 6 1/4 lb bolt in a closed bolt blowback shotgun firing slugs.

    Damn, all I needed was another project... Maybe even reduce the bolt weight by using a striker setup with a 1lb bolt and a 4lb firing striker to get some of the benefit of the whole open bolt inertial advantage that you guys were talking about earlier.
    Don't forget the spring tension too.

    Mark

  5. #55
    Gunco Veteran [486]'s Avatar
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    Quote Originally Posted by Markp View Post
    Don't forget the spring tension too.

    Mark
    Oh yeah... And the force to recock the hammer, the ruger mkII relies pretty much solely on that...

  6. #56
    Gunco Member carnut's Avatar
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    I think I slept thru all this in class are you guys rocket science guys or something. this much info is giving me a headack. I am impressed with this whole topic!!!

  7. #57
    Gunco Member akidynoken's Avatar
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    Quote Originally Posted by carnut View Post
    I think I slept thru all this in class are you guys rocket science guys or something. this much info is giving me a headack. I am impressed with this whole topic!!!
    impressed as well

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