Pow Pow! Blam!
I’ll keep this section short and sweet, since it has a limited relevance to the game. Firearms exist in the setting, but they are illegal for civilians to possess, and bringing a gun or trying to bring one into an XCrawl event is grounds for an immediate Disqualification (and probably an arrest, too).
Still, if for some reason someone has their hands on a gun, here’s how they work: take a regular ranged weapon, add 1 to the Proficiency Bonus, and give them the High Crit and Clip(n) special properties. Oh, and don’t forget to call it a gun. Bammo, conversion complete.
Clip(n): A weapon with a Clip has n number of shots before it has to be reloaded with a minor action. Typically d4 weapons have a clip of 9, d6 weapons a clip of 6 or 7, and d8+ weapons have a clip of 4.