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Thread: An idea for multi caliber conversion! (like the Gepard) :)

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    Happy Camper hcpookie's Avatar
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    Default An idea for multi caliber conversion! (like the Gepard) :)

    While working on my AKSU build I had an idea as to an effective way to make a caliber conversion like the Gepard with swappable barrels.

    This is for pistol-caliber conversions. Blowback is assumed, but a gas system could be added. Assume a "mostly standard" AK design.




    I want you to think of an AK barrel that doesn't press-fit into the trunion. Or if you put a Bulgy barrel against a Hungarian or Romanian 23mm trunion - it just "rattles loose" inside. Hold that thought.

    You start with this new trunion which could be as simple as a rectangular block with a hole drilled in it and riveted to the receiver (like a Bizon trunion?). The trunion becomes the "mounting frame" like an Uzi. The front of the trunion has an alignment notch, pin, or slot machined to interface with the rear mounting block - read on:

    Again, like an Uzi, the bullet guide is modified to be taller so that the barrel comes to rest AGAINST the front of the modified bullet guide. A matching notch is added to both barrel and BG for alignment purposes.

    Build a "rear mounting block" that mounts both the handguards and the top cover. This will be similar in concept an AKSU style (or M92 style) rear mounting block, and could even have the top cover and handguard notch added to it. An alignment notch must be added to be rear which should be in contact with the barrel alignment notch section. A new mounting block is required for each additional barrel in the "kit".

    The barrel is headspaced by adjusting the barrel's front-to-back location inside the rear mounting block. Once headspace is set, pin the rear mounting block in place on the barrel. The barrel and rear mounting block will stay together for the life of the barrel. Repeat for all other barrels in the "kit". At this point, the top cover and the handguards should be able to fit properly.

    Insert the barrel into the receiver and set the location for the bullet guide. The bullet guide can be inserted into a groove or slot that is held in place by the barrel, OR screwed in place, or any other idea that can make it removable. The bullet guide and barrel should have a notch cut that mates the parts together so the barrel stays set in a certain position in relation to the bullet guide - this will ensure it stays centered.

    NOW HERE'S THE FUN PART:

    Measure a CLEVIS PIN, or something similar, and drill/ream the barrel as if you were doing so for a barrel pin.

    Now, insert the clevis pin into the barrel pin hole and secure in place. Repeat for the other barrels in your "kit".

    PRESTO! you have a pinned barrel that is easily swapped. The calibers of choice may need a different bullet guide (e.g. 9mm vs. 7.62x25, etc) and so the need for the BG to be easily removed.


    Another way to look at this is to look at an Uzi, and instead of the screw-on barrel cap, you have the clevis pin. Instead of the "shoulder" on an Uzi barrel, you have the "rear mounting block".




    Considerations

    Remember, we are talking about 9mm as a basis, so we're not talking sub-MOA accuracy!

    A captive clevis pin would be required so as to keep the pin from wiggling out.

    The trunion and rear mounting block must be milled with a mating slot, or notch, or locator pins, or some other way to keep them aligned. Otherwise the barrel would not be able to stay sighted in properly beyond about 50 yards.

    The described configuration will make the barrel stay in its location via the the notched bullet guide and the notched/grooved rear mounting block. That is three hard points to secure the barrel, counting the clevis pin.

    The clevis pin should be made of good-quality steel... not an aluminum pin or anything like that. I do not know what the off-the-shelf pins are made of, but they should be tool-grade steel to ensure it doesn't get peened from repeated firing.

    The rear of the bullet guide could extend downward to create the front lip for the magazine lugs. That would allow for an easy change of magazines if different calibers called for different magazines. Otherwise, most magazines could be swapped as is already done for Uzi caliber conversions, among others. In fact, PPSH magazines may be able to serve the needs of all the interested calibers - 9x19, 9x18, .40SW, 7.62x25

    The bolt would be a one-piece design and could be a simple block of steel with notches machined, dremeled, or filed, for the rails and the ejector groove. Perhaps an Uzi bolt would be a good starting point... it is already drilled for a firing pin!

    Any required barrel extensions (typical pistol barrels aren't 16" long) would be needed for each and every barrel in the "kit" that isn't already 16+" long.

    The clevis pin could be replaced with any securing device - a bolt with threads on one end of the trunion would do the same task, but would require you to have the screwdriver or allen wrench as a part of the kit.


    A different design would forego the slip-fit barrel in favor of a threaded insert, if the trunion was threaded. Each barrel sleeve would need to be "timed" so the headspaced barrel had the rear mounting block properly set to TDC. Otherwise, the mounting block could be deleted from the design and it would be replaced with a collar that is more akin to an Uzi design.



    I'm still crystalizing the idea in my head, so there may be other considertations I have yet to realize. This, IMO, is a good start in the right direction!
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    Gunco Veteran muttman's Avatar
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    hcpooke,
    it sounds good to me. the clevice pin should be a sholder bolt comes in standard sizes and the shank ( the part that holds the barrel) is a +/- .001. muttman

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    Gunco Member jimmy_stikx's Avatar
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    I've been mulling around the beginning of something similar to this, but more in line with the FAL design for multi calibers. An insert pressed into a standard trunion that has a threaded stub the sticks out with an alignment notch. Hadn't gotten as far as the BG though.

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    Happy Camper hcpookie's Avatar
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    Hrm I posted a response but it didn't show... trying again...

    Muttman, I don't think the barrel pin needs to be a precise fit because the barrel is precision fit at both the bullet guide and the groove/lip fitting between the trunion and the "sight block". The barrel pin will only need to keep the barrel from flying forward, and therefore I'm thinking a clevis pin, or some other easily-removed pin.

    The PKM quick-change barrel feature is accomplished by a side-to-side moving wedge that is mated to a groove in the PKM barrel. The barrel is changed by sliding it back to a stop, sliding the locking wedge in place (presumably with the tip of a bullet because it would be hot!) and closing the top cover. That is basically my idea, except the locking points would be located at the bullet guide and the rear sight.


    Now, you could drill a vertical hole for a 2nd pin that holds the barrel pin in place. If you put it at the right location and made it long enough, it could be held in place by the top cover, like an AK-74 side-folder hinge pin is held in place. The 2nd pin would serve to keep the barrel pin in place and you wouldn't have an ugly cotter pin showing on the outside.

    Or machine a PKM-style wedge and do away with the barrel pin altogether. Hrm...
    Gunco Member #10

    http://pookieweb.net


    The "original" Boltcutter Rivet Squeezers:
    http://pookieweb.net/AK/rivet/boltcutters/boltcutter.htm


    Project Pink - the Pink and Blue AK-74:
    http://pookieweb.net/pink/pink.htm

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    Gunco Member jimmy_stikx's Avatar
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    Isn't that how the Bren barrel is held in place a retracting wedge thingie? I've never held one, only seen the pictures of the gun.

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    No Hope For Me 1biggun's Avatar
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    HC I did not follow all of it but if you are going to make a new trunion why not make it for a threaded barrel and just thread in what ever barrel you like and set it with a jam nut kind of like on a savage bolt action. I like the hand built trunion idea. I had thought of making up one for a 9mm conversion a year or so ago. I am working on a way to swap yugo trunions/ barrel assemblies with out undoing screws.( still looking for a couple more trunions)

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    Administrator sniper69's Avatar
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    pookie, I know an FAL is a different rifle platform - but would it be anything like the meeper setup? Here is a link http://www.falfiles.com/forums/showt...ghlight=meeper
    Searching for keyword meeper at the falfiles on a search encompassing everything a month and older will bring up a variety of threads on this.

    If nothing else maybe it will give some ideas
    "To show you how radical I am, I want carjackers dead. I want rapists dead. I want burglars dead. I want child molesters dead. I want the bad guys dead. No court case. No parole. No early release. I want 'em dead. Get a gun and when they attack you, shoot 'em."
    Ted Nugent - speaking at the NRA convention April 17, 2005

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    Gunco Member jimmy_stikx's Avatar
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    That's the one I was thinking of.

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    dyi
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    Happy Camper hcpookie's Avatar
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    Well hot damn! It looks like azhonkey did it, but with an Uzi trunion that screws down instead of a pinned-on barrel assembly.

    That is Twilight Zone *SCARY* that I thought of this and presto there it is!
    Gunco Member #10

    http://pookieweb.net


    The "original" Boltcutter Rivet Squeezers:
    http://pookieweb.net/AK/rivet/boltcutters/boltcutter.htm


    Project Pink - the Pink and Blue AK-74:
    http://pookieweb.net/pink/pink.htm

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