Bigbuck,
Couple of things here. One, you're getting into what is undeniably a form of advanced handloding. That's true anytime you load for an automatic of any kind. Service Rifles (like the AR) have a number of things you need to be aware of in order to create proper ammo. The floating firing pin has already been mentioned, and it's a valid point. Use a primer that has a relatively hard cup, in order to reduce the chances of a slam fire. CCI makes a #41 primer that's a duplicate of the primer that Lake City uses in all military 5.56mm production. The primer (whichever type you choose) must be seated properly. That it, below the case head and bottomed out firmly within the primer pocket.
Sizing. Small Base sizing has already been mentioned, and it's usually the best way to go. Most ARs have fairly generous chambers, and conventional Full Length sizing die will almost always give reliable function. However, the extra reduction provided by the S/B dies also assist in giving easier extraction upon firing, something most folks are unaware of due to the nature of the rifles. But it does ease the stress on extractors and can increase longevity of some parts. The real issue here, is headspace. With a bolt gun, clearance of .001" or so is perfectly adequate, and will allow the rifle to fucntion reliably. This is not the case with autoloaders. Here, you want a minimum of .003" of shoulder setback, and .004"-.005" is even better. The best way to check this, adjust your dies, and verify that the cases are being sized properly, is with a case gage. The simple chamber type gages like those from L.E. Wilson or Dillon, or the more complex RCBS Precision Mic, either will work. Bottom line is, you need to know that the dies are setting the shoulders back sufficiently, without going too far. Yes, you need a gage, and you need to use them, frequently. Doing so will eliminate many of the potential problems, that you'd really rather not run into to begin with.
Crimping. There's absolutely no need to crimp in an AR, provided you have sufficient neck tension to keep the bullets from moving during feeding. Rarely seen this happen after loading and firing tens of thousands of match rounds for ARs over the years. Crimping generally degrades accuracy, and the more you crimp, the more that degradation can be expected. This is why the 77SMKs have such a shallow cannelure on them; to meet the military requirement that they be cannelured and crimped, without really doing much of either. In otherwords, the absolute minimum they can get away with, and still be within "specifications." They have to do so to meet a government spec. You don't. Don't do it if you don't have to and your accuracy will thank you. If you decide you must, just think, "less is better."
Autoloaders aren't bolt guns, and several of the techniques needed to load them are quite different than what you may already be familiar with. Take the time to learn about these differences, and you can avoid some potentially serious pitfalls. The Sierra manual has a section devoted to loading for gas guns, but probably the best single source is Glen Zediker's book on, "Loading for Competition; Making the Target Bigger." In it, Glen describes many of these areas and does a superb job of explaining exactly the ins and outs, and most importantly, the "whys" of many of these operations. Good book, and one that needs to be read by someone just getting into loading for gas guns.
