Federal brassWhat brass? Because there's a world of difference between Lake City and Winchester brass.
For Varget, Hodgdon's .308 Service Rifle data shows a starting load of 43.0 and 44.0 max charge.
IME gas guns are just different and won't handle the same charges as your bolt guns. The service rifle data is geared more toward the M1 and M14 rifles... But probably a good place to start with an AR-10/.308
Can't help you with AR-Comp. But I bet someone else can chime in.
Mike