They are definitely tumbling. But, that group in the center dot is a decent group considering they are tumbling. Usually the bullets will miss completely if they are tumbling. For some reason, your bullets are tumbling right before they hit the target, which leads me to believe that they are somewhat stabilized for the most part, but at the target, they are under the minimum velocity right before hitting the target. If they weren't stabilizing due to incorrect twist in the barrel, they would start tumbling long before the target, and would probably miss it completely. Try to chrono the load.