As I'm usually the one getting everyone mad, I'm going to try a different role today.
SETTLE DOWN EVERYONE!
Ok, now that that's over, let me say a few things.
Excilefromhell, I don't think that anyone has really tried to discourage you, but they're really bringing up good points. Full auto, while fun, is really useless to the average Joe. It wears out barrels faster than you can think it, and it's awfully expensive to feed,even with a good reloading setup. I'm not talking hundreds of dollars, I'm talking thousands if not tens of thousands of dollars per year in ammo, if you want to have any fun for more than a day. You'll go through a barrel a couple of times a year. Is this what you want?
Ok, so I'm being a little extreme here, but if you're like me, you like to play with your toys. In order to really have fun with a toy like that, I'd like to be able to shoot about 1K per weekend, maybe once a month. I think that that's reasonable. If I were REALLY into full auto, I'd probably want to go through 1-2K every weekend, using several different guns to spread out the wear. However, I'll continue with my "reasonable" path.
Ok, so 1K per month, times 12...that's 12K which if the barrel isn't worn out yet, I'd be surprised. Expect it to go around 8K I'm guessing if loads are "standard" the twist is slow (1:12?) and you keep the barrel somewhat cool. Basically, expect a new barrel every 7 to 8 months at that rate. Barrels run about $200 not installed. At this rate, you might as well get the tools and do it yourself, I hear it's not rocket science at all. Surplus .223 sells for about $125 per 1K, so that's $1500 per year on ammo, and if you're reloading, buying in large bulk for everything, plan on being close to that, too. My .223 match ammo is more, but not by much. Let's not forget wear and tear on the rest of the rifle. Sure, it can take a lot, but new springs and action parts will eventually wear out. I'm guessing at least $100 per year in upkeep...at least.
Now, honestly, if I were going to shoot that much, I wouldn't be using an AR. I'd buy a Ma Duece and really have a good time. If you're going to go full auto, make it BIG. Alternately, a GE Mini Gun would be nice. I think my ego could handle that.
But seriously, the folks I know that shoot full auto, that's their major shooting interest. They buy several types of full auto guns, usually i the same caliber, and shoot them all, usually at LEAST once a month. Full-auto isn't really an on the side kinda thing. It kinda takes over your interests. If that's you, go for it. Look into .223 caliber AK's (AK-74), Galil's, and other .223 caliber full-auto rifles...that way you can spread out the wear.
What's all this going to do for you? Well, if you get bored with shooting up a fridge or something (I'd get bored with that in about 2 seconds) then you're going to want to start practicing shoot and move drills. Make sure you keep "safety first" when you do things like this. If you really practice hard, take a few classes, and work at it, you'll be a force to be reconed with within 100 yds or so, but without a team, you're still dead meat. This is where the stuff the other guys were saying comes in. If you're fighting lone CQB against several aggressors, you're screwed. The only way to really do any damage in a SHTF deal, is to be far away, and not make more than one shot at a time before you move, unless you have a very good position, and you know you can egress long before your enemy gets near you. (Imagine a cliff, and an easy get away for you, and your enemies are in front of the cliff) Now as Art says, "Nothing will be like you imagined" so this is purely anecdotal. Anyway, in that position, sure, I'd take several shots, but I'd need a rifle that's accurate at 500 yds, maybe more, and I'd have to know how to use it. A full auto AR from this position would be a remake of an "A-team" fiasco.
That's what these guys are getting at. Full auto tends to be a waste for the average Joe. Good luck in your purchasing!
Dang! I've still got almost 6k more characters to type before I hit the limit!