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One of the most frustrating maladies that can afflict auto and semiauto firearms is the doublefeed. When we usually speak of a doublefeed, we are refering to a failure to fully extract the fired case prior to stripping the fresh cartridge from the magazine.

Some thoughts on this problem come to mind:

1) During firing, pressure causes the case wall to grip the chamber. A rusty, pitted or irregular chamber surface can lead to a failure to fully extract.

2) A weak extractor spring, poor extractor design or a weak case rim can add to the likelyhood of troubles.

3) If the action of a locked breech firearm begins to open while the gas pressure is too high, the case will offer increased resistance to extraction. This assumes that the wall area times the coefficient of case/chamber friction will be greater than the area of the case head.

4) Shortening the barrel of an otherwise unchanged gas operated action will reduce the dwell time, during which gas is diverted to operate the action. This is why simply chopping the barrel of an AR15/M16/CAR to the front sight block is not a good idea, unless you move the gas port.
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