In case you didn't know, I'm not popular with the crowd that worships at the altar of mil-spec. Yes, it does guarantee a certain level of gear, but in a lot of instances, it is really old technology. Why do I mention this? Many people ask me, "What is the best ready-to-go AR-15?" I don't name a mil-spec worshipped brand. I simply tell them, "Almost any brand that has been through the Sweeney Checklist."
So, what do we check?
Is your gas key tight? This is the little spigot on the top of the carrier. Pull your bolt/carrier assembly out of the upper receiver. Grab the key and try to move it. If it shows the slightest bit of wiggle, you're cruising for a malfunction. If yours is still tight, look closely at it. Is the carrier staked? This means that the screws that hold it on had the edge of the hole peened over to trap that screw in place, immovable. If not, get it peened.
Now, there is one company, Yankee Hill, that does not stake AR-15 gas keys and tells you that staking voids the warranty. Since I have never seen a loose YHM gas key, I'll go with that for now. The first time I see a loose one, though, I'll go back to my default position: You need to MOACKS it.
The Mother Of All Carrier Key Stakers will make sure your key never comes loose. You get one from my friend Ned Christiansen. Fair warning; I've known Ned for decades, we're friends, but even if I didn't know him from Adam, I'd recommend this product, because it works. He makes at least three different versions, so you can't complain about cost.
Is your AR-15 extractor on the job? Here's how you check. Since you have the carrier out, take the bolt out and remove the extractor. Is there a small rubber donut wrapped around the extractor spring? If not, your AR-15 is soon going to be asking for time off. You need, at the very least, a rubber o-ring. Available from many sources, it is the cheap version of the real deal and proper product: the D-Fender.
The AR-15 has been grossly "under-extractored" from Day One. The recent Army-recommended gold spring, the stronger one, isn't enough on its own. Get a D-Fender, or if you can't justify the cost (geez-o-pete, it is only ten or twelve bucks. One mag of ammo costs more than that.), then get the O-ring from Brownells.
Install it (either one) on not just carbines, but rifles, too. Yes, I know the Army tells us that rifles don't need the O-ring. Well, the Army is wrong. Can you have too much tension with the super-duper gold spring, the black stubby insert and an O-ring or D-Fender? I haven't seen it yet.
Does your AR-15 have a 5.56 chamber?
The difference between the .223 and the 5.56 is in the leade. (Pronounced leed) That is the distance the bullet has to jump out of the case neck before it hits the onset of the rifling. This is also called freebore. On the 5.56, the freebore is twice the distance that the .223 provides or more. The 5.56 is loaded hotter, and thus gains more velocity, but controls the extra pressure by having the bullet jump further before it hits the rifling. Shoot 5.56 ammo in a .223-lead chamber, and you risk popping a primer. Maybe not now, but soon.
A popped primer can and will fall into the lower receiver and cause all manner of malfunctions. I even witnessed an AR-15 with a .223 leade, used with 5.56 ammo, that fired a three-shot burst when the user attempted to push the selector lever back to "Safe." And this was a semi-auto AR-15, not a select-fire version. Luckily for him, the lead instructor was looking directly at him at the moment, and could see that his finger wasn't on the trigger. And he had further luck in that the crew and I were on hand to diagnose the problem.
Solution: Ned again. Ned makes his 223/556? Gage and a specialty reamer. If your AR-15 chamber fails the gage, that is it has a .223 leade, the reamer will ream the leade and only the leade and make it a 5.56. It will not cut the shoulder, and can't change headspace.
"No problem, my barrel is marked 5.56." Guess again. We have, in LEO patrol rifle classes, run into a number of rifles and carbines where the barrel was marked "5.56" that actually had .223 leades.
With certain brands, you won't have a problem. But with many, the marking is no guarantee. The gage is not too expensive, but the reamer is, admittedly, pricey. However, ask around with AR builders, and you'll find someone with the set who will be happy to gage and then ream if necessary.
Oh, the chrome in your mil-spec barrel? Yes, it will cut that off, but that's where the chrome gets torched off first anyway. If you have a thousand rounds down the pipe, your leade probably doesn't have much chrome left anyway. Melonite? It can't be cut. If you have a melonited .223 leade, you are just plain out of luck.
Hammer and Trigger Springs
In every class, we tell the assembled AR-15 users, "OK, here's the proper way to assemble the hammer and trigger springs. At least one of you will do it wrong before class is over. It might be you, so don't be too hard on whoever it happens to."
No, really, it is a simple thing, but we regularly see rifles in class where it has been done wrongly. There are a veritable fistful of mnemonics to remember how they should fit, but a picture is much better.
On the hammer, the spring loop has to be tight up against the hammer. I use a demo hammer with a silver Sharpie line to demonstrate. On the trigger, the front loop has to be under the front nose of the trigger, not the shoulder of the trigger bow. And when it is assembled, the hammer spring legs have to be above the trigger pin and outboard of the trigger spring loops.
Anything else is wrong and asking for trouble.
Stakes are not just for the grill. If you have an AR-15 with a telestock, is the castle nut staked? If not, then whoever put your carbine together did it wrong. Staking is where the takedown pin retaining plate is whacked with a sharp punch, in order to push metal into the small notches in the castle nut. (Not the large notches, I've seen a few where the castle nut was put on backwards. Oops.)
If the castle nut isn't staked, the nut can vibrate loose, the tube can wobble, and it can even allow the buffer retainer to pop out and tie up the firing mechanism. Don't say "No, it can't." I've seen it and more than once.
Now, there's one exception to this, and it is because of the "tacticool" gear trend. Those who have replaced the regular AR-15 plate with one meant to be a single-point sling attachment ring will often find the new plate too hardened to be staked. In that case, use red Loctite, but do so knowing that you'll have a heck of a time getting things apart when you finally come to your senses and ditch the single point sling.
I admit it, I'm pretty opinionated when it comes to gear. I've busted a lot of caps, seen a lot of competitions and taught a lot of shooters, both LE and non-sworn. In that time, I've seen just about every way an AR-15 can be broken, mis-assembled, over-modified, built cheaply and otherwise messed up.
Take care of these five things, and you have eliminated probably 99 percent of potential future malfunctions on your AR-15.