You You might want to consider making modular BOB's.
The bag you want for, I don't know… Say a burning chemical rail-car that clears out your neighborhood for 24 hours, while you all sit in the high school gym, is not the bag you want for "Katrina", or the bag you want for "Mad Max"/"The Day After" etc.
I would put any weapons into the module where the level of emergency warrants it, or even a "weapons module" of un-PC items that could cause you grief in a smaller scale disaster that you can secure more easily from the other survival items.
Each module has a progressively higher threat level, and a greater time/distance before you're in a region where law/order and relief exists, the lowest level being just a day or two, with highest level being indefinite/never. And as the time and the "threat" increases, the worry over legal consequences and securing firearms or weapons properly begins to fade in the face of the danger of not having them.
If you live in a normal "nice" area with nice, normal people, sitting for 1-2 days in a shelter, that handgun is probably a liability, if not at least just a extra needless worry over it being discovered. If it's a matter of days or weeks with looting, the situation is reversed, NOT having that handgun is the liability.
The "neighborhood disaster"/3-day bag is probably best filled with clothes, underwear, insurance papers, cash, toiletries, prescription meds, etc. I'd leave most food, shelter, and first-aid items out of it. It's wasted space and weight. I would probably even include a radio, (more for entertainment than news) playing cards, a book, things to pass the time. Assuming your community is better than the human flotsam and jetsam of New Orleans you can safely rely on the presence of food, shelter, and medical items from friends, family or the surrounding community.
Or even less dramatic, you'll just be sleeping on friends and relatives couches for a few days, or a hotel, before going home. If you've got a CCW, bring your gun on your person as you would daily anyway, just realize you might be sheltered in a prohibited place like a hospital, a school, or even federal property. If you don't have CCW, I don't foresee that things are more dangerous from a crime standpoint than any other day.
The "city-wide disaster"/1-week bag is where you want to look into things like food, water or water purification items, first-aid, fire starting, heat, cooking, and shelter items. This is the level where a weapon starts to make the most sense (above and beyond daily CCW if you have it.). There's still the entire state and region you can rely on for assistance, but it will likely take a few days to get going because of the magnitude of getting assistance and law/order to the entire region. Mild to moderate looting, and crimes of opportunity are more likely.
The "state-wide disaster"/1-month bag is what you'd want for a Katrina-level disaster. It's not too different than the 1 week bag, other than the duration, and you want to emphasize more on the food/shelter/water/fire/first-aid items. Extended looting and unrest makes a long-arm a good idea at this level.
The "Global/National SHTF" "forever" bag? I can't help you there. I have no idea. I doubt it's possible without massive training, and after that, you'll know what to pack better than I. IMO, your only real chance is to "bug in" and live in the right place beforehand. You need an amount of supplies and food you can't conceivably carry, probably even in a vehicle.
You can address the modularity by packing the bags for each additional level. Some of the military-style bags like Maxpedition or Blackhawk type brands, and even some of the camping/hiking ones have additional bags and modules you can strap together in any configuration you might want. Many of these systems have a smaller detachable small backpack or waist-pack that would serve well as the lower level pack, or you can just figure out modules out of the bags you have on hand.
) I don't live in a flood plain either. No mountain is going to slide down on our home... There's a few latino's in the farming community, but I won't call'm gangs. A Tsunami would have to travel 20 miles up stream to get to our small town, still living well above the river, I have nothing to worry about. Although we're close to British Columbia, I'm not worried about an invasion. (B'tween me and the neighbors, I think we can take'm anyway). 
