Ahh… I have not worked with the slingshot yet, but I suspect you might be having a similar problem that I did with the arrow about 3 weeks ago. They had just recently opened up the ability to switch out the static mesh of the arrow in first person view, previously it was attached in the animations.
I believe you might be experiencing a similar situation, and if that is the case, you need to export the slingshot skeletal rig, and swap your mesh that way. I was unable to do this because I had an incompatible version of maya, and imports and exports continued to get erros.
Hmmm. That would explain why it’s not attaching the projectile to the mesh. I expected I would pick the rock socket, but that doesn’t work. If noone else has any other insight, I guess I’ll just let this one ride.