Any of my guns that have much recoil at all, must be held in both hands, resting the back of my left hand on the front bag, but holding the gun tightly, as if offhand shooting at game. This gives identical POI as offhand shooting.
To rest the gun itself on the bag, ie: BR fashion, is to open groups and to shoot high.
My little Savage Weather Warrior .375 shoots 3 1/4" high and 1" to the right if shot in BR fashion.

edited - I should have mentioned I have to shoot my double muzzleloader as noted above or it crossed 1 1/2" and shoots different elevations x 1 1/2" as well. A 'hard' hold is necessary to make it regulate 3/4" to 1" apart at 50 yards.
I hold similarly to 404bear, except the left hand is rested on the bag.