I have 2 rifles that cover all the scenarios I can think of that are most likely to happen.
1] Home invasion. This is a criminal situation. This is a situation where they have the element of surprise. I have little to no time.
Saiga 7.62x39.
Simple, robust, reliable and powerful. Cartridge is useful for home defense - can easily penetrate interior walls as well as cabinets, furniture and other obstacles between the lead I'm launching and the scumbag(s) waiting to receive it. I am 100% the opposite of all the THR advocates of lesser-penetrating ammo. My motto is - over penetration is an asset, not a liability.
No fancy lights, no fancy reddots. No tactical-ninja garbage. Nothing to get in the way. Nothing to slow you down in your time to engagement. 1 simple operation - charge it via charging handle and rock n' roll. That's it. Point and shoot. Home defense distances are short, real short unless you're Bill Gates. No need for optics. No need for sights. Situation will largely be a display of quick and blinding violence. Reality is, you'll be sighting the weapon like a shotgun and pulling the trigger.
http://www.thedenverchannel.com/news/8287436/detail.html
Here's a home invasion scenario that really happened and was discussed here on THR a while back. Note that that a lot of the shooting occured when there was a victim(s) in one room with the invader in a hallway or other room and shooting taking place through a door.
It is critical to find a cartridge that will go through **at least** one door and still have enough juice left to take out the invader who is hell bent on killing you.
2] Riots, gang wars, looting. Post-natural disaster.
Colt 6520 (16" superlight) w/ Aimpoint (4moa, 50,000hr battery life) and BUIS - 6 USGI magazines loaded with 75gr Hornady TAP (210 rds total). Side arm - Springfield XD Tactical, 9mm, loaded with 115gr +p+ Ranger, 16 + 1.
In these situations, there's some warning - sometimes very little, sometimes days or weeks worth. Either way, there's enough time to get the rifle, strap on your ammo, turn on your red dot and get ready.
AR-15 is fast to shoot, quick to reload and is accurate. In this scenario, shots greater than 50 yards may be possible depending if the rioting mob is into Molotovs or is armed with firearms. If they have firearms, they are fair game at any range. The key here is speed in engagement, speed in reloading. Red dot rules here for fast engagement. You may have 5-7 thugs trying to burn your house down from all directions. You must be able to engage 360 degrees, rapid fire and from distances that might be in your face to 100 yards out or more. So a 0-200 yard platform is ideal. Move and shoot. Rifle should be short, light and ergonomic. Thugs will be moving and moving fast.
Having an optic that is night-capable is essential as you won't be going to bed, but be on guard and alert.
For a post-natural disaster, this serves well. Lightweight, and can engage targets out to 200 yards no problem if you need to leave your home to search for food/water/fuel. Also works well if you need to evacuate the area by foot if the disaster is substantial.
If you choose to stay home and defend it - will work well against any looters or thugs, in close or far away. Lightweight is ideal as you can keep it on your body for the duration without getting tired.
So bug in, or bug out - you're good to go. Night/day - good to go. Short duration, long duration - good to go.
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For revolutions, uprisings, civil wars etc...well, that all depends on what your strategy is - defensive/offensive. IE, are you going to stay out of the way and just protect your butt, or are you going to be a participant? If a participant, in what role? Will you go around and assassinate political opponents or others who you deem to be on "the other side" or aiding or assisting the other side's movement?...or join up with gangs/militias etc..and participate in raids, ambushes or other activities?
There's other situations, like post-nuclear war. Massive outbreak of disease. Super-natural disaster like meteor strike etc....
These situations are above and beyond just firearms choices. In these situations, you are going to have to weigh them by how long their effects will last. If it is years or decades, then your firearm strategy must be compatible with that. This gets more into survivalism. You have to gauge how society will react to it, and for how long. Outbreak of disease could be pure and total chaos where it will be everyone out for themselves. This could be really bad. Not that nuclear war, meteor strike or anything else couldn't result in people defaulting into savages...it is the duration of chaos that is critical. That effects the choice of rifle, cartridge etc..