Help needed building an onager

[P. Clodius Secundus]

To clarify Robert,
I meant to indicate that the spring bundle was probably heavily pretensioned using a stretcher machine similar to that used for other catapults. That is where the real power comes from. Twisting the washers to build tension like many people do is a case of diminishing returns. The more you twist the less responsive the sinew bundle gets over the long arc (90+degree) of its travel. Even though the initial load is greater the arm won't continue to accelerate as well. At least that's the theory. Pre-tension good, Twist bad!
For the arm's shape I'd go with a gradual taper. Lighter/longer until it almost breaks is better. A gradual taper also causes les grain violations that encourage splitting. From what I've seen you don't need to worry too much about the spring gripping the arm or holdinng it upright. At high tensions it will soon crush the fibers of even hardwooods like oak, ash, or hickory enough to make it's own gripping surface and it will center itself up just fine under load.