Faults uncovered are divided into two groups: those that require immediate correction (or at least correction before use) and faults that can be overlooked in an emergency, where the fault would not impair function sufficient to prevent use as emergency equipment. An example of a fault in the first group would be a rifle lacking sights.
There are very few faults that fall into the latter category, as a rifle as emergency equipment is not like a fire extinguisher that only has 87% of its charge. A fault in the second category would be a stock that is present, but loose. Usually, most faults on firearms are an “all or nothing” situation, especially defensive firearms.
Properly done, an Internals/Selector check takes two minutes, and you should get into the habit of doing this check every time you pick up your AR, or any other firearm, for that matter. I know, I know, you did it last week, and the week before, why do it again? Because since the last time you may have been doing some work on your AR, got interrupted, and left it as-is. Having now forgotten, if you don’t do a check, you may be depending on an inoperative rifle.
We’re already at the field-stripped stage, right? If not, move back up, read and field-strip your rifle.
Inspect the lower.
Make sure the hammer springs are on either side of the trigger pivot, and above the trigger pivot spring. Check to make sure they are intact, and one or both legs are not broken. Look at the hammer and trigger pivot pins, and make sure they are flush to the outside of the lower, and not protruding. A pin that sticks out indicates it is not secured, meaning some spring in there is out of place. Move the selector back and forth from “Safe” to “Fire” (and on to “Auto” or “Burst” if the rifle/carbine in question is a select-fire weapon) and visually inspect the lower to ensure there are no debris, dust, lint, threads from cleaning patches or loose primers present.
Inspect the carrier. Grasp the carrier in one hand and the carrier key in your other hand. Try to move the key. Any movement at all means the rifle must be pulled from service until the key can removed, reinstalled and locked in place.
Inspect the bolt. Are the locking lugs clean and oiled? Any visible chips or cracks in the lugs require the rifle to be pulled from service until the bolt can be replaced. Does the ejector move in and out of the bolt when pressed with a small rod? (It won’t move under finger pressure.) Does the extractor flex when pushed? Too much or too little movement requires service on those parts.
Finally, pull the bolt forward in the carrier, and stand the bolt on its head. If the weight of the carrier causes the bolt to collapse towards the tabletop, the gas rings are worn and must be replaced.
Install the bolt back into the rifle and close the action. Close the upper to the lower and press the takedown pins back in place.
Selector Check, SEMI
Lock the bolt back. Inspect the chamber. Once clearly unloaded, press the bolt hold-open lever and allow the bolt to close under its own power. Move the safety to all positions. If it does not move, the problem must be found and corrected.