He has some great videos. I watched this one and was impressed at the calculations, the transparency and the celebration at the end when he reached supersonic speeds.
I think the idea is that at the end the sling starts to unwind too, increasing the effective arm-length (that non-linearity is one of the reasons trebuchets are so efficient to begin with). As a result it's possible that decreasing the spool to near zero is like putting the gears of your car too high. Reducing the spool size ensures there's enough torque left to put more power into the final swing.
I suspect that with a little iterative modelling of the whole system with derivatives, efficiency could be bought from ~40% to ~80%, giving you an extra few hundred mph...
I'm pretty sure the only reason the video trebuchet wasn't a proper bullet is because it's made out of plastic instead of a ball bearing. Aren't most handguns subsonic?
He has some great videos. I watched this one and was impressed at the calculations, the transparency and the celebration at the end when he reached supersonic speeds.
I would think you want it decreasing to near zero to extract all the kinetic energy from the mass, leaving the mass stationary as it hits the ground.