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Originally posted by Oblivion437
Well, I've not had the chance to get under the hood of an AR-15 at all, but I've gotten the general impression that it blew it over the moving parts directly rather than using a conventional piston. Is the gas ever in contact with the extractor?
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Nope, it is completely isolated from the extractor and the ejector, both.
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Well, think what you will of my sources, it does use the same round, which exerts the same pressure, but the shorter barrel results in greater pressure on the action, thus increasing the rate of fire ever so slightly. ~1 more round a second over A2-A4 variants of the M-16. The operating method of the XM-15 series results in carbines having to operate under greater mechanical stress, thus increasing probability of reliability falires.
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The speed can be tuned by adjusting the size of the gas port in the barrel, the additional round per second in the M-4 is a result of having to work against a stiffer, shorter buffer spring.
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The M-4 variants are carbines, the AK-108 is a rifle. They operate and are constructed differently around these principles. A number of my fellow gun lovers have informed me that there is something of a 'barrel threshold' for M855 or other SS-109, that with a 1:7 twist (the optimal twist) on that round, you need 16" of barrel to give appropriate velocity for regular and reliable fragmentation at longer ranges. Given the general operating method of the AK-107/108, long range shooting is (finally) an applicable notion.
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The M-4 operates identicaly to the M-16, it is given the name "carbine" only in defference to it's shorter size. As I said before, the M-4 displays acceptable fragmentation out to 200 meters, which is plenty for the needs of the US Army.