Playing With Toy Guns

I've always wondered what I would do when my son began to want to play with toy guns.  We've never had toy guns, but we knew he would eventually be introduced to them through some friend or relative.  The truth is toy guns was always fun for me growing up, but I've also seen other kid take it too far.  After all, kids are now playing realistic and violent video games and grow up enjoying violence.  So, when his cousin introduced him to playing guns today, I was presented with three options: (1) tell him "no guns" while his cousin played on, (2) let him just play and learn his cousin's rules, or (3) teach him my rules by playing with them.  Option 3 is the best method.  It is the only kind of lecture I know that always sinks in.