Principles of Propulsion
Well, the principle seems to be sound.
It's an easily pointed, sort of stable platform that will propel a projectile having enough mass and velocity to inflict enough harm to stop an attacker.
I can think of some improvements, though.
The projectile is awkward; too long, easy to fumble.
Let's see if we can shorten it.
Of course, once it's that short, it would have to be made entirely of metal to be heavy enough.
The energy for propulsion is stored in a bent piece of wood/metal/composite, and reloading for a second or third shot means bending it again. That's very slow and inconvenient.
Perhaps we can come up with a way of storing propulsion energy in a sleeve of some sort, along with the projectile. It has been said that certain powders store energy quite well, releasing lots of it when ignited. So, a sleeve housing the (much shorter) projectile, some burnable powder, and some kind of sparking device to ignite it.
A projectile sleeve of this kind (let's call it a canister), would be small enough that one could have several of these in a hopper of some kind, that could be fed into a launching compartment, and that would make sending subsequent projectiles an easy matter.
Considering that there is no longer a long, bent crosspiece, some attention could be placed on directing this new, shorter projectile. After all, it would have no feathers to stabilize its flight, so perhaps a tubular shroud of some kind would ensure that it flies where it's directed.
I have no idea what the resulting device would look like. Try to imagine a machine with no string, no crosspiece, a tube of some sort for directing projectiles, a hopper affixed containing the projectiles, and some kind of striking device to ignite the burnable powder (like a small hammer, I suppose).
It doesn't sound very elegant, but it seems to me it would be more effective.
I wonder if one could obtain a Patent of Invention for such a thing. *Sigh* Probably not. There are too many mechanical problems to solve. Really, it's more a task for a Swiss watchmaker than an armorer.
Well, it was a nice thought -- a pleasant distraction.
Let's get back to the task at hand, shall we?
Now, place your foot here, and grasp the string with both hands, like this . . .