Press the selector to SAFE. Press the trigger with 10 to 12 pounds of force. The hammer should not fall. If it does, the trigger mechanism is defective, broken or improperly assembled, and the fault must be found and corrected. Release the trigger.
Press the selector to FIRE. If the hammer falls when the selector is moved, but before you press the trigger again, again the mechanism is either defective or improperly assembled and must be corrected. The selector function when on SAFE acts by blocking movement of the trigger. If the hammer falls, the selector did not prevent trigger movement, and when it was moved to FIRE the trigger completed that movement, allowing the hammer to fall.
With the selector on FIRE, press the trigger. The hammer should fall. If it does not, it is obviously not ready for issue, use in a match or for defense and must be corrected. If it does, now hold the trigger back. Grasp the charging handle with your other hand and cycle the bolt. Gently release the trigger.
You should feel it “click” as the hammer resets off the disconnector hook and onto the sear nose of the trigger. If there is no click, open the upper receiver. If the hammer is forward, or still caught on the disconnector, the mechanism is either improperly assembled or the parts mis-timed. Find out why, and correct it.
If the rifle passes the SAFE and FIRE checks, you must then check for disconnector retention during cycling. Cycle the charging handle to cock the hammer. Press the trigger and allow the hammer to fall. Hold the trigger back. Briskly pull back and release the charging handle, allowing the bolt to close under the full power of the buffer spring. Gently and slowly release the trigger.
The hammer should “click” from the disconnector to the trigger sear. If it does not, it must be adjusted. Then leave your finger off the trigger and briskly cycle the charging handle and allow the bolt to crash home at full power. Then dry-fire again. If the hammer has followed the bolt forward, the disconnector is (again) either improperly installed or mis-timed and must be corrected.
All those faults are covered in their own chapters, so read on.
Inspect the fit of the upper to the lower. While a certain amount of looseness is often customary, and does not hinder proper function, an excessively wobbly fit indicates either serious wear or a dimensional mis-match. Excessively loose upper-to-lower fits can be solved by exchanging uppers until the uppers and lowers you own all fit properly.
I have this discussion often in the law enforcement classes: While “mix-and-match” rifles will almost always function properly, remember that the accuracy of a rifle goes with the upper, not the lower, but the felt trigger pull goes with the lower. A matched upper and lower used in a precision marksmanship role must not have the upper and lower swapped with patrol car issue rifles, or the accuracy and/or trigger pull of the precision marksmanship rifle will be lost.
Inspect the stock for tightness. Solid stocks (A1 and A2) should be immovable. Tele-slider stocks will usually have some wobble in the sliding portion, but the buffer tube itself must be tight. Loose A1/A2 stocks or loose tubes in either rifles or carbines must be corrected.
Inspect the barrel for tightness in the upper receiver. An upper with a loose barrel cannot be accurate, and the cause must be found and corrected.