What brass? Because there's a world of difference between Lake City and Winchester brass.Using sierra 155s I have Ar Comp and Varget. GRT showing Varget would be a little less pressure and more velocity. Any idea where to start with a gas gun? Looking at 45ish gr of Varget and 43ish gr of AR COMP.
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