In short, you are going to want to grip the gun same as you would in the field. From a bench, holding the gun in both hands, with your forearms resting on a sand bag best simulates firing off hand. If you rest the gun on the bags, it will be steady, and may give tight groups, but it will not hit in the same spot as off hand, as recoil will tend to bounce the gun off the bags. Least this method has worked for me for years with heavy recoiling big bore revolvers.
Larry