Just stole this from another forum in the UK.

kernel gadaffi

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Minuteman
Mar 2, 2012
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Wolsingham, County Durham, UK.
My friend was on the phone last night, telling me he was out zeroing in his 308, he started at the target at about 20 yards , firing 3 shots , didn't even hit the paper, he moved closer and fired another 3, still nothing on the paper.......

........only then did he realize he had been firing 243 bullets..................
 
I had my kid do that to me one day. He managed to load (understand, he was 24 at the time) a 5.45x39 into a stack of 7.62x39 in a magazine on me. His response? "But Dad, they're all green and they look the same to me."
 
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).