The Precision Reflex 15 inch GenIII DPMS .308 Carbon Fiber Forearm is currently not available.
The manufacturer does not state if the Precision Reflex 15 inch GenIII DPMS .308 Carbon Fiber Forearm is a DPMS high or low profile. This is probably because the top rail is not designed to meet up with the receiver. For more information I recommend contacting Precision Reflex.
I believe the Rock River Arms LAR-8 is an Armalite Style of AR10. So the Precision Reflex 15 inch GenIII DPMS .308 Carbon Fiber Forearm will not fit. However, I recommend contacting the manufacturer to confirm.
This will put the gas block right at the end of the forearm, completely under forearm depends on the gas block and size. The overall length of the forearm is 15.75”
Yes, this handguard will fit that upper.
Yes, it will include one 8.5" rail for on top and three 3" rails for the sides and bottom.
As stated in the description.
The ID of the forearm is 1.9in and the OD is 2.165in.