This reminds me of an encounter I had a couple of years ago at a local firing range.

A local basketball star showed up to shoot with a couple of his friends. He brought a collection of handguns just thrown into a backpack all together. No cases, no nothing, just a bunch of hardware clanking together. There was some junk in there to be sure, but also some really nice hardware. A couple of Kimbers at least. Anyways midway through my shooting session one of his friends asks if I think I can help him out. He had shot once and the gun he was shooting jammed. I took a look at it (it was a very nice custom Kimber 1911) and asked him how many rounds he put in it. Eleven, he said. Now this is a single stack .45 and should hold 7 rounds max. I ask him to show me the bullets he put in and he shows me a box of 9mm. After assuring that the gun HAD fired (I asked 3 times) I was able to clear the jam and open the chamber. The extractor had captured the round exactly right and the firing pin had struck the primer dead center. He was really lucky that the round didn't jam in the barrel on it's way out.

I showed him how to read the side of the gun and the box of ammo to make sure he was putting the correct ammo in whatever gun he was shooting.