I use a layered system... each layer takes everything from the previous layer and adds something.
I generally carry most of the parts that go into an ordinary altoids tin kit about my person in every day carry, so I don't carry a centralized altoids tin kit.
My every day carry consists of the general stuff (wallet, keys, cell phone) but also a small first aid pouch on my belt, a spare wallet lots of useful items in it, a small mirror, lighter, a couple of pocket knives, LED keychain light, surefire light, multitool, and so on.
My smallest survival kit is a cargo pocket sized SAS tin thing. It contains all the basics- This is the minimum I'll carry if I know I'm going out into a situation where it might be needed- for instance, hunting, hiking, geocaching, and so on. It's also a component in the next few layers.
The next step up is a fanny pack, vest, or small backpack. It can carry the SAS tin, but also additional items that won't fit into the tin that would be useful, so this can be considered a survival kit expansion pack. Things like 550 cord, GPS, shelter, bug repellant, mosquito headnets, canteen w/ cup, belt knife, energy bars, LED headlamp, etc. Basically a "packing light" setup specialized for wilderness survival. While the SAS tin is the minimum I'll carry when I know I'm heading out of civilization, this is what I will usually carry.
The next step up is what most people call a bug out bag, but go bag, get home bag, 72 hour kit, and so on. It's basically a medium sized backpack that might be a little too heavy for everyday hiking, but carries all of the above mentioned gear, and more of what I might need if I have to go anywhere on short notice. A change of clothes, hygene gear, more food and water, larger first aid kit, and so on. The idea is not paranoid militia "the government is going to turn on you", SHTF, TEOTWAWKI, PAW, and so forth- the idea is you just have something ready to go incase you need to leave in a hurry -- for whatever reason.
Say there's a hurricane coming. Maybe your house is on fire. Maybe your 7, your father is in the military, and he has been called up and has to leave with only a few hours notice, and you have to go to one of his friends house so they can watch you for a few days. Maybe construction workers uncovered an old WWII 500 lbs bomb buried in your backyard and everyone in the area needs to evacuate for a few days. See what I'm saying? The idea is that you don't find yourself in tom cruise's situation in war of the worlds, where you run in and go "we're leaving this house in 60 seconds" and have to scramble to find things you might need. Instead, you just grab your bag and you're ready to go. Now, you have a little time left over to think about anything extra you might need that fits that particular situation. If your house is on fire, you grab your bag, and have plenty of time to think "I should grab these valuables, or my lockbox with personal files and information in it" on the way out. More importantly, for people with families, getting them out is first priority. You don't have much time to screw around thinking about the long term when you're feeling your way through smoke trying to get family to safety, so you do that ahead of time by making a bag and just grabbing it when you need to go.
The bugout bag contains all the components from the previous levels, so you are ready for wilderness survival if you need it, but the point is it's not limited to that. It's for a wide spectrum of possible scenarios. Far more likely, you will look for a hotel to stay in for a few nights if you have to evacuate. Or a friends/relatives place. Or even your car on the side of the road- but if you DO find yourself in the wilderness with nothing but your bag, you're ready for that too. For those living in areas where you can predict the most likely situation, like in hurricane or earthquake prone areas, you can tune your bag more towards that and it will still be useful in other situations since you still have the basics covered.
The last level of course is the vehicle kit. Basically it combines all of the previous levels, but also contains vehicle specific components to keep you running. More water, wool blanket, tool kit, spare parts, spare tire, can of fix-a-flat, tow ropes, compressor maybe, chains, battery starter, etc.
I didn't speak of guns because that doesn't apply to everybody. If you have a concealed carry permit, you carry your pistol as part of your every day carry and since each level incorporates the previous, you'll have it for all. Long guns can be introduced where appropriate given the situation.
