The world is mostly TL 3.
With a good and well-researched excuse I might allow one level of High TL. (Alkenstar, Andoran, possibly Gnome Artificer, possibly Dwarven Artificer/Scholar, Musketeer.)
Only people with the Demolisher or Musketeer templates can use guns safely. (Use the rules from HT:85-96 instead of/in addition to the rules in the Pyramid articles.)
Non-Demolisher/Musketeers gain the following condition as long as they carry a gun:
Reverse Luck (roll three times and use the worst result when firing/loading/repairing a gun.)