Tilt test,
check head spacing,
Check trigger function
fix the loose op rod guide (hammer and punch)
fix the op rod if it no longer moves after fixing the guide (lots of them come bent)
inspect the gas assembly for cracks, ensure the piston can't jam sideways in it.
lock tight the rear site retaining screw
I've bolded the things you may want to check before firing it so that you can return it.
You may also want to check:
- the barrel index: is it vertically in line with the receiver? (the front sight may be out of wack to compensate, or it may just be out of wack by itself). It is misaligned in any other way?
- does the bolt roller make contact with the receiver when in battery (you should be able to slip a piece of paper between the roller and the receiver -- with the op rod removed)
- is there good contact between both bolt lugs and the receiver lugs when the action is closed (i've been told to check this with a spent casing in the chamber, as it may appear loose when the chamber is empty)
- is there good negative pressure acting on the gas piston when the rifle is tilted muzzle end upwards (with the op rod, op rod spring and spring guide removed), such that the pistol descends slowly (desirable) or the piston just drops (not desirable)
- does the gas port in the barrel line up with the gas cylinder (can be checked with a round toothpick or thin allen key and a flashlight)
- ensure that the op rod is not bent and that it and the bolt slide easily back and forth when the rifle is tilted up and down 30 degrees from horizontal (with the op rod spring and guide removed) -- this is the Tilt test
- the rear sights are often loose or shake loose with repeated firing because they are either poorly made or not tightened adequately at the factory. This isn't a deal breaker since most people replace the rear sight for a USGI version. There's also a fair bit of oil under the rear sight when new, which sprays backwards when shooting it for the first time. Wear your eye protection or hold a cloth over it
- does the bolt hold open work (some don't lock open on an empty mag). Again, this may not be a deal breaker if you decide to swap it out for a USGI or aftermarket product
- ensure that the safety bridge inside the receiver effectively stops the firing pin from moving forwards until the bolt is fully seated into battery.
