Keep your AR-15 from malfunctioning. Check these 4 key things in 30 seconds or less and stay in the game.
If you have half a minute, you can confirm that:
1) Hammer and trigger pins are flush to receiver sides and not hanging out. A trigger pin sticking out almost certainly means the hammer spring is improperly installed and is not detenting the trigger pin in place. The result will almost certainly be sporadic or continuous misfires, burst fire, shut-down. A hammer pin sticking out probably just means it was not pushed in far enough. It is detented by the “J” spring, which is simply a length of spring wire that is permanently staked into the hammer. These practically never fail or come loose.
2. Hammer spring is correctly installed with both legs horizontal, spread out against inside wall of receiver, and laying on top of trigger pin properly in that pin’s outboard groove, acting as a detent for same. A hammer spring installed backwards will give a light primer strike causing misfires, and although it may appear to be laying on and detenting the trigger pin, it in fact will not. This will lead to the trigger pin walking out and causing failures to fire, or doubling/burst fire. Hammer springs not installed backwards can still be improperly installed, with legs either under the trigger pin resting on the floor of the receiver, or inboard of the receiver wall and thus not laying in the detention groove of the trigger pin. In either case the above trigger-pin-walking problems will be the eventual result, plus, when the legs are under the trigger pin instead of on top of it, the blow to the firing pin is reduced somewhat as the spring is not as “wound up” as it would be when properly installed.
3. Carrier key is not loose. Simply hold the carrier in one hand and try to wriggle the carrier key with the other. This is almost certainly the number one cause of AR-15 malfunctions. Carrier keys come loose, allowing gas to escape from between the carrier and key. Then there is not enough gas to operated the bolt. The immediate, field expedient fix would be to simply tighten the screws (9/64, and sometimes 1/8, Allen wrench). A better fix would be to remove the screws, clean them and dry them, apply red Loctite, and tighten. Better yet, when time allows, is to do the above and then stake the screws in, displacing carrier key metal over them. This is supposed to be done at the factory but most manufacturers are doing it poorly and some are doing it not at all. Even staked, screws have been known to come loose and although they cannot separate from the carrier key due to the stakes, they will actually turn and lift the key off the carrier. One final bit of insurance after staking and Loctiting can be had by counter-staking the screws, just to the clockwise side of the stakes in the carrier key. This way, if the screw ever did try to turn, the outwardly displaced metal of the screw will hit the inwardly displaced metal of the carrier key, preventing the screw from turning.
4. Firing pin retaining pin (cotter pin) not blocking the firing pin. Simply slap the carrier’s back end into your palm to make sure the firing pin cannot come out. Also, with the bolt pushed into the carrier, you can press the firing pin forward and check that it protrudes from the bolt face. With this check you have checked two things: that the firing pin is free to travel fully forward and that the firing pin tip is present (although I have never, ever heard of one breaking). Note that the firing pin will not protrude if the bolt is extended forward.
This article is an excerpt from Gunsmithing The AR-15.