If there still remains any questions..... here is one last quote fromRandy Fritz........(actually it is pretty much exactly what I had originally said)
Not to mention your own testing proved his point exactly...
PAUL MOHR said...
shot was about 4 or 5 inches high
RANDY FRITZ SAID....
"Slugs are so slow that they're still traveling down the barrel as the gun recoils upward and to the left," he explains, due to the torque of the heavy projectile as it spins out of the barrel's rifling. "If you sight the gun in at the bench without holding the fore-end, you'll actually sight it in 5 to 6 inches high and to the left of where it will shoot when you hold the fore-end normally in the field."
Therefore, says Fritz, you'll have to adjust your bechrest technique. Instead of using your left hand to plump up the sandbag as you would with a centerfire rifle, clutch your gun's forearm firmly to duplicate your hold in the field.