Been there...done that. A 243 in a 300 Win Mag. Both rifles looked/are the same (a long action with a blocked magazine for the 243). I was working up a load for the 243, my son grabbed the rifle from the safe, and I headed out to my back yard shooting range. I hand chambered the round, squeezed the trigger and an odd Bang/Fssst/whoosh sound happened. I pulled the round and it had split all the way down to the rim. I went back into my basement, checked all my load data, checked the powder, pulled a bullet and checked the powder weight....all good. So (like a dumb-ass) I went back up stairs and tried another round. Click...no fire, lift the bolt handle, reset the trigger. Click....Bang/Fssst/whoosh sound. Split another cartridge. I then looked down at the Scope and thought, "thats odd, the 243 does not have a lepould scope on it". Then, and only then did it occur to me I had the wrong rifle. I hate to admit it, but it does happen (but it has never happened to me again).