[martineta]

You sure you didn't have a cartridge that ever so slightly crept over into supersonic velocity. Reason I ask is I loaded some 1050 fps 220 matchkings, shot nicely in the backyard at elevation under 200 ft and 80 degrees. I went up in the mountains a few months later and shot some at 3,000 feet and about 50 degrees colder and was getting supersonic cracks.