High power bullpup crossbow from SPEAR GUN rubber

Discussion in 'New project ideas' started by Mongolbow, Jan 18, 2014.

  1. Mongolbow

    Mongolbow New Member

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    I am planning on making interesting project.... try to use 16 or 19 mm spear gun rubber in design like bullpup crossbow, to shoot bolts with it. what do you all think is it dooable and what parts need to be reinforced? Looking to get around 150-200 pounds of draw. Thanks for help
     
  2. beaverIII73

    beaverIII73 Junior Member

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    From my experience with X-bows I would say the trigger assemble and the front forks. And at that wait of draw you will want to think about a way to cock it. Good luck.
     

  3. JoergS

    JoergS Administrator

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    If you plan to shoot bolts with spear gun rubber, then I recommend you make your own bolts as they will have to be very very heavy. Spear gun rubber is slow and performs well only with heavy projectiles. Commercial crossbow bolts (today they are usually fletched, therefore technically those are short arrows) are lighter than arrows, not heavier.

    Think very thick wooden shafts and massive steel tips, weighing at least 300 grams, maybe as much as 500 grams.

    Spear gun rubber needs to be cocked either with a winch or one strand at a time, you can't do it with a stirrup, at least not on a narrow fork.

    Forget rollers, the friction at the draw weight you are looking at will be staggering, negating the whole draw length advantage. You would also have to build these rollers so sturdy that the weapon would be very front heavy. You can make the weapon foldable for easier transportation maybe, but rollers, no, I don't see it.

    If you just want to shoot a heavy commercial bolt, you'd achieve better speed at much lower draw resistance by using Thera Band rubber. Then you could also cock it faster and employ rollers.
     
  4. Mongolbow

    Mongolbow New Member

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    Wow thanks for advise... will try to stay away from the spear gun onces... draw weight doesnt scare me... as I have cocked 250 lbs custom c bow before. (195cm 95kg You get the idea) but I definitly dont want to shoot 0.5 kg spears. Regular to Heavy bolts (same weight as store onces) is what I want to shoot with it. I will go with ThereBAND, but where do I get it? and what would be best rubber for max possible velocity.

    ps: Great work on all your stuff JoergS. recently discovered your youtube channel and gotta say impressive stuff.
     
  5. JoergS

    JoergS Administrator

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    You see, on a narrow fork, the angle of the string at the sear would be almost 360 degrees - much more steep than on any crossbow out there. Don't compare rubber based crossbows with normal crossbows. Also don't forget we are NOT talking compounds here. There is NO let off. Very few people can cock a 250 lb recurve crossbow with bare hands. But a slingshot crossbow with narrow forks is more comparable to a 350 lb recurve due to the sharp angle. It pinches your fingers badly.

    Thera Band Gold can be found on ebay. Just use my band calculator on my www.slingshotchannel.com (under FAQ). Double those numbers for crossbows.

    Good luck!
     
  6. Mongolbow

    Mongolbow New Member

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    Yeah wouldnt be able to draw 350 pounds for sure....

    Ok great found theraband gold... Gonna go to hardware store get all components and start on stock.. Thank you very very much.
     
  7. Mongolbow

    Mongolbow New Member

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    One more question please....

    how much theraband do you think i will need and how many layers I gotta put it in? double? triple? or more?
    Thats regular theraband gold sheet right?
     
  8. Mongolbow

    Mongolbow New Member

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    cant find schematics anywhere
     
  9. JoergS

    JoergS Administrator

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    Again, just use my calculator. It all depends on the projectile weight, the draw length and the desired compromise between band life and power.

    For crossbows use twice the dimensions the calculator outputs.

    http://slingshotchannel.com/band_calc.html
     
  10. Mongolbow

    Mongolbow New Member

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  11. BoyntonStu

    BoyntonStu Member

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    May I suggest the following design?

    [ame]http://youtu.be/QuRBu8OWmFU[/ame]

    My next step is to build bolts out of 1/2" CPVC pipe about 9" in length.

    I will use a heavier spoon broadhead and insert concrete or sand to adjust the bolt weight.

    Momentum is your friend.

    A baseball pitched at about 120 fps was lethal to over 100 batters

    The spoon arrowhead shown in the video was too weak and it bent on impact.

    I am designing a new trigger mechanism to increase accuracy and to set the trigger pull.
     
    Last edited: Jan 19, 2014