Answer:
Trebuchet
Explanation:
I've imagined building a Trebuchet. After that idea, there were some things to consider carefully on my project.
The payload should be deadly and also win the air resistance, which brings up the fact the speed and the acceleration must be produced by a great torque by the Trebuchet's counterweight.
My medieval arm is rigid and launches payloads at a steady position so that the loss of forces could be minimal. A stationery trebuchet at the shooting is vital for a greater range, as the speed and impact of its payload would play its role as well.
No friction on its parts (pivots, joints, guide chute) so that we can improve the effectiveness of constant shots.
In addition to this, some parts of the Trebuchet must have no significant mass.
Finally, at each shot, the acceleration of the projectile could be calculated differentiating the speed of the counterweight over time.
All of these matters ought to be considered, for the prototype