Might be a parallax problem at 50m. Next time you're there, crank it up to 20x, sandbag the rifle, and move your eye around while looking through the scope. If the crosshairs move on the target when the gun isn't moving, the target and the reticle aren't focusing on the same plane. Focus your scope until the movement stops, and then see if you can shoot a good group at 20x. Setting the scope at the 50m mark might not put your AO in focus at 50m, sometimes those numbers are off.