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4. No apparent movement of bolt or carrier regardless of gas setting
a. Check that the gas plug is in the “A” position.
b. You DID remember to put the gas piston back in, didn’t you?
c. Check to make sure the gas tube is pinned in place and has not rotated. Ideally, the exhaust ports in the gas tube should be at 8 o’clock and 4 o’clock but if they are at 10 and 2, it will not affect operation.
d. Ensure that the gas port is not obstructed.
e. Check that gas piston is not undersized or worn. Proper diameter is between 0.429” and 0.431”.
f. Excess leakage around gas tube, see No. 6
g. Check that the bolt carrier “rat tail” is straight and in the recoil plunger detent and not jammed against the back of the lower receiver when closed.
h. Take the gas piston spring out and roll the piston on a flat surface to check for straightness. Reinstall the gas piston with the bolt and carrier removed. The piston should fall freely through the gas cylinder and gas nut. If not check gas tube and gas nut for roundness and damage.
i. Check for cracked gas block.
j. Make sure you haven't put a metric gas plug in an inch gas block. A metric gas plug is about 3/16" longer than the inch plug. The gas plugs are not interchangeable.
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