The other day I came across a fascinating video about an “assassin’s crossbow”.
https://www.youtube.com/watch?v=eM9t3Zk4KCs

What particularly interested me was the cocking method, which used the principal of a helical thread to gain mechanical advantage needed for drawing the powerful prod.






It struck me that this principal was also proposed a century or more later for cocking airguns.

First there was the twist handle air cane proposed by Lanes Bros. ca. 1890, which may or may not have gone into production, but for which no surviving examples are yet known. This was discussed in a previous thread (https://www.airgunbbs.com/showthread...ghlight=pistol)





The thread pitch (turns per inch, or tpi) is much larger than that used in the crossbow, and so the mechanical advantage would be less, but on the plus side it only took four twists of the handle to fully compress the spring.


Then there came the twist-grip pistol patented by Clarke, Johnstone and Fearne in 1923:





Now the thread pitch was even greater, and spring compression was achieved by only ¾ of a turn. As a result of the loss in mechanical advantage this meant that the pistol had to use a relatively weak spring and a short piston stroke and so was quite low powered.





I very much like the convenience of this cocking system in a pistol, and to get round the low power problem I had toyed with the idea of lengthening the piston stroke, and using a ratchet system, so that the pistol could be cocked with more rotations. Then I realised the obvious from the crossbow and walking cane examples: you do not need a ratchet to hold the spring compression when you release your hand pressure if the thread pitch is high enough.

This set me to wondering what the minimum pitch would need to be, for this to happen? I suppose there are equations for working this out, but this is beyond me. I would probably have to resort to trial and error using some models, unless anyone has any better suggestions?

It makes you wonder if there is anything that is truly new under the sun.