Were these weighed powder charges, or dropped? If they were dropped, maybe your drop messed up and threw a too light charge that left velocity below the speed needed for stability. Thats a pretty long bullet, and I imagine it needs a certain amount of speed. My .475 shoots sub 1" groups at 50 yds. with 1350 fps. loads. I slowed them to about 850 for a "plinker" load. Shoots about 3 FOOt groups with these. Apparently too slow for this bullet.