workinwifdakids said:
I'd say in a calm fashion, "Wow! You have a <insert firearm here>? I've been looking at those. Mind if I take a look?"
Then, after looking at it enviously (even if you have two already), hand it back to him and say, "Well, we wanted to stay for a while longer, but our rule is that if we're shooting with people we don't know, there has to be a rangemaster. If you're not cool with that, we'll shove off, and no harm no foul. On the other hand, we'd like to stay, but we need a rangemaster. Are you okay with that?"
That way, you allow him to be in charge, and he's actually forced to make the decision you wanted anyway. Then, you walk over and lay out some rope as the firing line, and -- the rest is safe history.