I just did a few no CW dry fires on my treb. Seems to work right, though a few things need adjustments, and needs something to prevent it from falling over forward. I think I just make sure it is not on an uphill slope, and stake the back down or something like that.
As I have progressed with this implementation of the design, I have realized several flaws in my machine that should be changed for future ones. Changes can be made to tolerate elastic parts better at the cost of arm drag for example that would be good for a flimsy machine like mine, and least until I get some less stretchy string. Also, before the CW detaches, the transfer of energy may not be as complete as I want because of the same stretchy string and some minor building errors/choices. Again, the next one will be better in this respect.
I'm quite worried about the sling. With the axle around 3.5 feet off the ground, and a 4 foot sling, things could get ugly. 7 feet of string slapping the ground/treb! Also, the tuning literally chooses the release angle, though the first throw will basically be at a random angle, and the next one should fix it, but a random angle on such a machine is bad as it might just slam the sling and projectile into the ground!