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So if my gas tube(or any of the gas system parts) fails, I basically have a bolt action ar15?
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Yes, but that is true of ANY gas system-type...when parts fail, they don't function.
One of the advantages of being an issue weapon with a major military power is that weapons get constant use and evaluation. Just through attrition (of equipment AND people), unsatisfactory designs and sub-standard parts get tossed as designs are refined and part-quality improves. By that process, the current AR-pattern 5.56mm rifles (from quality manufacturers) are reliable and safe in function.
The biggest issue with the AR-pattern gas system is that is just moves the gas so damned far using what it probably one of the most fragile function-critical parts of a current-issue combat weapon...that damned gas tube!
The tubes rarely fail in/from use (from my experience), but they are exceptionally fragile and can be damaged if the rifle falls from a not-to-high location, in the right orientation, onto a hard surface...and they can be easily damaged by hand, too (unintentionally or intentionally).
Examples I have seen:
M-16 falling off of a stairwell landing and hitting the middle of the top handguard on a handrail below when the soldier was wounded in the hand and dropped his rifle.
A heavy-barrel AR-15 upper being cleaned w/handguards off and the owner picking the upper off a table by the barrel and losing his grip. As he reached for the falling upper, he caught it by the gas tube which pulled out of the upper receiver as it folded then snapped off at the front sight base.
Injury and inattentiveness aside, damage like that shouldn't really even be possible in a military weapon...I mean, come-on, you can put an M-16 out of commission with your bare hands for Christ's sake!!!...but I can't argue with the success and lifespan of the design.