Problem is home 3d printing is still a pretty weak plastic and plastic has never been a great medium for high quality paintball markers, other then for shrouds and shells. You could use wax replacement casting, but again, cast markers are pretty low tech as well.
There are plenty of the required cad files floating around for autocockers and other of the more common paintball designs and you can buy some base extrusions to mill. I think that's about as open source as it gets. For the price of buying a high quality 3d printer you might as well by a CNC mill and go to town.