See the sniper on sniper scene in saving private Ryan. Us cross dominate and lefties have managed for years. Break action singles and doubles are 1 way to go. Levers and pumps work nicely too. For shotguns I love the BPS, its patriarch the Ithica 37 isn't bad either.
As mentioned before safeties are the biggest issue. There are a few models that have reversible safeties. I believe the Marlin lever guns still have a half **** "safety" as well as the button,on exposed hammer guns. Savage has a top tang safety, at least my old1does. I also just picked up a small ring Mauser w/a safety lever built specifically to be left handed that has a right handed twin part. Once fitted they take less than 5 minutes to swap out.