I vote he was disassembling it, didn’t make sure it was unloaded, and when he pulled the trigger to remove the slide, sent a round through his hand. Then made up the drop story to save face.
This would be my first guess.
I heard of a story a few years ago where a guy was disassembling his Glock and didn't check to see if it was loaded or not. When he pulled the trigger to get the slide off, he shot his daughter.
