cocking device for bullpub crossbow

Discussion in 'New project ideas' started by LW, Oct 17, 2013.

  1. LW

    LW New Member

    As I wrote in the Thread “Brutally short 25mm steel ball crossbow” I made bullpup crossbow, with a cocking device inside the body of the crossbow.
    Sadly, this thing only exists on paper and in my head…:eek:
    I do not know why I've never built this thing?

    But here are some pictures of my design and the function…


    Top view relaxed

    Top view stretched
    So, 50 cm elongation of the bands produce one meter extension.


    What do you think, should I go on…?
  2. Obl1v1Aus

    Obl1v1Aus Meh!

    That's a really cool design LW. I'm just curious how does it "chamber" a projectile. The pics look like its gravity fed, have you considered a sliding bolt mechanism to load a round after cocking?

  3. Slagskimmer Mike

    Slagskimmer Mike thinks TBG smells better than roses

    I like what I see here a lot. The cocking mechanism is a handle with two cords attached, if I'm seeing it right.

    @obliviaus---A sliding door in the magazine is a good idea , the one in the m10 hexnut gun design works great, and could tie in readily with the cocking motion.

    I guess the concern is how much energy the cocking mechanism will steal from the system as the slides return forward. If they some how get out of the way before the shot with a return spring/rubber--then no problem. I could just be looking at the print wrong.

    Hurling one inch balls with that much drawlength (safely) appeals to me very much.
    Lovely drawings, I hope you decide to pursue this.
  4. LW

    LW New Member

    You are right.

    Yes I had a sliding door in mind, but I am not sure if I want that door opening, when pulling back, or open it with another action after the cocking. This would be a kind of a safety, so you could re-stretch bands without firing the amo.

    As this is still a concept, but I don’t want the slides stay at the end after cocking, I would attach on each slide a small rubber, just too automatically pull the slide back, so there it will be the stopping for the cord. And even the cord hanging out at the end will look to stupid.

    This system was build for cal. 50 (12,7mm) and I believe 50cm draw length with strong bands would be enough…?!
  5. Obl1v1Aus

    Obl1v1Aus Meh!

    I was thinking of using a sealed bolt system on a similar project. so when the bolt is in the forward position the magazine is opened to a cavity in the bolt itself, but the breach is sealed. you would cock the rifle and then pull the bolt back, sealing the magazine and dropping the single round into the breach
    I'm trying to create my bolt in Inventor so I can 3D print it at work to see how well it works

    Attached Files:

    • bolt.jpg
      File size:
      27.9 KB
  6. JoergS

    JoergS Administrator

    The issue that I see here is the limited accel length. Even if you use 50 cm draw, then you will have to pull the rope up to 150 cm total length, all the way up to your chin. 50 cm is not very much.

    Will there be additional rubber bands to pull the slide back to the front after the weapon is cocked?

    I see some additional minor issues, like a canting of the slide (the rope attachment seems to be a bit off center). But those can be solved. By all means give it a try!
  7. beaverIII73

    beaverIII73 Junior Member

    I love the look and I think the design is very cool. I have one comment about the cocking machanism. If you use two soild rods with some kind of hook on the end you could push the rods back in before you fired the round. Then you would'nt have to worry about the cord being pulled back at a rapid rate or loosing any energy on the recoil of the sling.
  8. coyote-1

    coyote-1 Member

    Yeah, that was my first thought too. The other thing I'd do is a foot stirrup on the front; as an owner of 'regular' crossbows, I find that an incredibly valuable component of cocking a device with heavy draw weight. And certainly, for such a weapon you'd want it to propel that projectile as quickly as possible :)