Add-on for Blender to connect rigid bodies via constraints in a physical plausible way. (You only need the ZIP file for installation in Blender. Click the filename and at the next page "Download", right click and "Save As" won't work!)
When creating a default element group eg RC Columns, then saving the .blend and exiting blender, reopen blender and open the previously saved blend, no Element Groups are shown in the list under the Bullet Constraints Builder.
For now you have to manually store and load settings for the BCB in the scene, there are two buttons next to the Build and Bake buttons for that. I'll consider changing that in the future, though.
When creating a default element group eg RC Columns, then saving the .blend and exiting blender, reopen blender and open the previously saved blend, no Element Groups are shown in the list under the Bullet Constraints Builder.