I personally do not have an answer for ya on the life expectancy but as far as havin' a spare layin' around just in case ... I would certainly build a parts set for each weapon if it were I.
Unless your M14 is a real M14 and you use it as a SAW it should outlast a couple of barrels. Just use a light coat of gease of any kind. Too much of any grease will be a dirt and sand magnet. Op rods were not the big problem some made them out to be once we got the M60 for a SAW.
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