Re: Stupid, or Foolish???
I was out shooting a couple years ago with a group; I shot first while the newer shooter learned to fill magazines. After I ran through the first mag, I picked up the next and the weight felt off....looked closer and saw that I had a 1911 mag full of 9mm. That could have turned my day sour, but it was a good learning opportunity for the new shooters of what not to do.
For the OP, stupid or foolish depends on education. If they didn't know, then I'd call it stupid. If they knew better but went along anyway, I'd call it foolish. Either way, it's unwise.
While we're on the topic, has anyone ever seen a straightwall case with a 308 headstamp? Care to guess how that one happened?