I did some reading on AR kabooms last night. I just happened into a thread on the subject on ARFCOM and then pursued a few other threads.
I came to the conclusion that it is difficult/rare to overcharge a .223 case with enough powder to cause a kaboom (assuming you are using a correct rifle powder in the first place). There just isn't enough room in the case to hold that much of an overcharge. But bullet setback is a different issue, and CAN cause overpressure enough to cause a kaboom.
If they feed right, the bullet never touches anything.
And there's the kicker, "IF
they feed right..." What if they don't? A dirty feedramp, a softnose bullet that catches, a mis-shapen bullet nose, too loose neck tension, a defect in the feedramp ... I can see all kinds of ways things could go bad. Maybe they aren't typical, but they could still happen. A crimp might possibly help in these rare situations and protect your safety.
So far everyone says there is no reason to crimp. But is there a reason NOT to crimp? I think the added safety, however minor, would be worth it. I am new to reloading rifle cartridges - this .223 in my new AR will be the first rifle caliber I'm reloading. I want to do it right, and safely. I've been doing handgun reloading for about 1-1/2 years now. Not a whole lot of experience there, but I have a decent understanding of the basics.
I had one scary experience at the range. I reload for my .45colt lever gun and revolver. A friend also reloads for his .45colt revolver. It was HIS reload that gave me a scare. He had a few where he did not seat the primers all the way and they were hanging up in his revolver. He gave them to me to shoot in my lever gun. As I was shooting and loading his rounds, happened to notice a cartridge with a significant bullet setback on it's way to being chambered. Luckily my rifle is an 1892 model and you can see the rounds loading from the top. It was dumb luck that I caught this one. I asked him if he crimped his rounds and he said no. So I certainly won't be putting any more of his reloads into my rifle! I trust him to get the charges correct because he is very meticulous. But skipping the crimp step is just dangerous IMHO. It may not be needed for his revolver, but then, his cartridges were being shot in my levergun in this instance. Just because your AR has no feeding issues that might cause bullet setback doesn't mean that you won't hand a few of your reloads to a friend who's rifle might have a feeding issue. And kaboom he goes. I don't like the possibility of that scenario, so I think I want to crimp my rounds. That's just me trying to be safe.