I'll play. Parameters are "S&W revolver" and "bug out bag." I'm going to go with "realistic" instead of "fantasy" on this one.
Why bug out? It's probably a flooding situation. Could be a wild fire. Or a hurricane. In theory it could be massive civil unrest, invasion, or zombies, but the the former are much more realistic. Odds are it's a somewhat organized evacuation and you're on your way to stay with relatives, a second home, or in a shelter. You might need to protect yourself, but it's not likely to be a raging battle against a band of determined marauders. More likely it would be a street crime perpetrated by an opportunistic thug or desperate unprepared person.
If you do it right, your bag will already be pretty full and heavy: water, food, some clothes, ID/insurance/banking documentation, tablet computer/cell phone, flashlight, batteries, chargers. You'll probably be driving away, but walking or biking is a real possibility. Weight is definitely a big consideration. Where you're going to carry it is too.
Consider a big back pack with shoulder straps, chest strap, and waist strap. Most waist band holsters would not work that well. Carrying it in the pack is too slow to access. Ankle carry might make sense, but does it if you're walking several miles? I'm going to go with pocket carry as the best option.
My pick: Airweight J-frame in 38 special with a good pocket holster. Any of the Airweight models would work equally well. They're as light and small as they get and easiest to carry and conceal. I'm even going to go out on a limb and say that one with the dreaded lock would be the way to go. (Let's assume you've put a good number of trouble free rounds through it w/o lock issues.) There may be times in a shelter, at a family member's home with children, etc. when being able to secure the revolver would be desirable.