1) AK variant, like Ak-47 Bulgarian, etc. Built to take lots of punishment, and can utilize a 30 round magazine. Pretty cheap, really, tho I understand they are ALL going up up up. It looks pretty too :)
2) Some version of the AR-16, ie, semiauto of course. There are several that make fine examples in this line, Bushmaster, Colt, etc. Can also utilize a 30 round magazine. These are truly beautiful rifles but can be a bit more complicated than the old workhorse AK variants.
Use a .223 cartridge, the AK's usually use the 7.62 x 39mm round which is very inexpensive, about $90 for 1000 rounds, that's a lot of bang for the buck! Another rifle to consider would be the old M-1, or even the older SKS. Anyone of these rifles would serve you well in a situation you describe.
Always remember, safety first, and for goodness sakes PRACTICE!
Usually I would pick a gun for home defense and therfor short range (ie shotgun). But I'm thinking that civil unrest means two things:
1) Longer range - you don't want the target to get the chance to get close.
2) Rugged construction - you can't run down to the gun shop to get a replacement part.
Again, thanks all.