I wish I knew what happened also. In this particular case, I was somewhat concerned about shooting the loads in the model 60, but I took my Ruger 77/357 and shot a couple of rounds through it before I tried them in the smith. I figured if there was problem then the rifle would handle it better. The only other possible issue is that of the gun. I had purchased it used and had noticed I had difficulty chambering rounds in one of the charge holes on the cylinder, and that the barrel appeared slightly ringed just forward of the frame. Whether that was a factor or not I don't know, I have been reloading and casting bullets since the mid 70's and I have had cracked cases and head separations before, but never anything like this. I was surprised that the cylinder basically split in two. I would had thought that the chamber that blew would simply split not the entire cylinder. But by no means do I believe its a problem with the gun. I was using my reloads and accept the responsibility of what happened.