Summary

After seeing Zheng3's penny mortar idea, I thought that it would be possible to take a similar concept and make it into a ballista-like version. Until I print this myself I am not entirely sure if the tolerances are correct and everything is sized exactly. Give it a shot and I hope y'all like it!

Instructions

To make 1 Desktop Ballista Print*:

1-Ballista 2.stl

1-ballista slider.stl

at least 1-Ballista Projectile.stl

UPDATE: I just added a file that has all three parts in the same .stl

.stl measurements are in cm
50% infill would seem to be fitting

Extra supplies:

a few-rubber bands

Assembly...
1) cut a rubber band to make it into one strand
2)slide the band through the hole on ballistaslider.stl
3) take each end of the band and tie them to each hole on the ballista frame
4) slide ballistaslider.stl into the groove of the ballista frame
5) place ballistaprojectile.stl on the groove of the frame

...pull the slider back and launch the projectile...