Wedging the gun up against the shoulder and or the chest helps.
Yeah, that's about what I did. Strangely, I also find that the BFG is easier to hold without the armor on. It gives the MS more arm mobility, to support the gun. So I have my HWS using the normal rifle and funnels, and my old Nu (who, thanks to a parrot-related incident, is missing his V-fin) is using the BFG in sniping pose.
One complaint I have about the HWS, BTW, is the big shield. It's so heavy that the part that holds it on the arm can't stay up, and the shield sags.