Installing Sub-Panel in Detached Garage
You'll need minimum 1-1/2" rigid for MHF cable.  That means 8" burial depth to give 6" cover.  12” cover under vehicle drives.  

THWN or XHHW wires are a much better choice. It would allow 1-1/4 conduit and other wires in the conduit.  Cable in conduit is silly as it makes for a very hard pull.  The only reason to do it is if you wanted to transition from conduit to in-house wiring without any splices. 

A wired ground, or Rigid as the ground, is fine.  You need grounding rods also.  

Remember at #2 wire size the splice  box must be quite big.