According to officials at Colt, those reasons included the fact that six of the ten M4s drawn for the test did not meet the minimum rate of fire of 700 rounds-per-minute mandated under Mil-Spec IAW Mil-C-70599A(AR), which requires a cyclic rate of fire of 700 to 970 rounds-per-minute. The M4s used in Dust Test 3, delivered to the Army in June 2007, met mil-specs when delivered; however, together the ten drawn for the test from the U.S. Army inventory averaged only 694 rounds-per-minute.30 While performing comparably with the HK416, XM8, and Mk16 in all other respects, the M4 carbines used in the test experienced a large number of failure-to-feed and failure-to-extract stoppages.31 Colt says this is because of the sub-mil-spec rate-of-fire of the test weapons.
Colt also states that ATEC's testers were unfamiliar with the M4s' 3-round burst configuration which, depending on the position of the cam, will sometimes fire 1 round or a 2-round burst before firing a 3-round burst. This unfamiliarity, said Colt, led to single rounds and 2-round bursts being counted as stoppages. With the exception of the M4s, all other weapons tested were fully automatic with no 3-round burst provision. Further, Colt points out that the test itself did not meet Mil-Spec 810F and "was not repeatable."
In response to what Colt described as "the premature media reporting" of the raw test data, Program Executive Office Soldier suggested that Colt conduct its own extreme dust test. So Colt contracted a DOD-certified testing agency, Stork East-West Technology Corporation in Jupiter, Florida, to conduct its own dust test according to mil-spec guidelines. In this test of ten M4 carbines, which was conducted under a protocol identical to that used in Extreme Dust Test 3, only 111 stoppages were reported.
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