FWIW, I have found that if I place my front bag rest right up next to the front of my trigger guard, that My POI does not differ between shooting from a rest or off-hand.
Remember, the forestock is NOT supporting the barrel; rather, its hanging off the barrel. Depending on the fit of the barrel to the stock, and what weight RamRod you hang from it, you should not see any difference.
Most MLers have octagon barrels, either full length, or half length, and that extra metal is there to help support a stock that hangs from the bottom of the barrel, or half rib.
I found that shooters who ask me, and have trouble with their bench groups or patterns shooting high usually are putting lots of pressure DOWN on the top of the wrist with their trigger hands, attempting to control recoil so the gun does not slap them in the face. They have mounted the gun wrong, and have their face over the top of the comb, instead of alongside it. I correct the head/stock position, ( the stocks need more drop at heel) and then ask them to take their thumb off the wrist, and lay it along side their index finger on the side of the stock. We move the fore rest back to the trigger guard. An alternative technique is to have them put their thumb on the back of a trigger guard, if that is possible( Not usually on a Lancaster style trigger guard) and use a pinch type squeeze technique to fire the gun off the bench.
Some bech rest shooters find a spot about 8 inches from the muzzle and rest their long barrels at that point. I know they get good groups, with rifles, because they have found a spot where they interrupt the barrel vibrations ( Harmonics) the least for their rest. I don't know if this puts the ball at a different POI than if the gun was shot off-hand. Considering how little pressure these shooters put on either the stock or trigger guards, I doubt it.
Finally, always check the fit between the barrel and the stock. If there is visible play in the forestock, even in a shotgun, you may want to bed that barrel more firmly. Try putting paper wedges at the very front of the forestock to see what that up-pressure does to shifting the POI. If the results are desireable, then build a well with some modeling clay,and pour some epoxy bedding compound into the well, and set the barrel, covered in a release agent, into the stock and let it set up. When that dried, go back and re-bed the tang and breech area for at least 4 inches with the bedding compound. Leave an area around the lugs and keyways of at least an inch on either side of it, so that the lug and pin, or key continue to work as designed.
