I too doubt that it was a 50 bmg at that range even the X bullet would have failed or the dog would have misted. Could have well been an AE,Beowolf or S&W in 50. I did see something much like that w/25-06' w/100 gn hps. It was a large jack rabbit nothing left but and ears everything else was just liquified .

Sierra HPs cirica 74'.