Rigged models end up stiff in Gmod

For my first attempt at rigging, I tried doing a headhack of Francis from L4D onto the N7 Armor from Mass Effect. When I compiled it and tested it out in Gmod, the model ended up stiff and unposeable instead of ragdolled. I’m pretty sure this has something to do with the collision mesh, since I don’t really know how to make one of those. In StudioCompiler I set it to use the reference model as the collision mesh, and I’m assuming that’s why the model isn’t working, so can someone tell me how to do collisions for ragdolls?