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Saco Defense: M60
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Saco Defense was bought out by General Dynamics, and hasn't made M60s for several years. The Marines switched to the M240G a while back, and the Army is in the process of changing to the M240B.
Bushmaster made a run of about 3500 M4s during the Gulf War, which Colt objected to. Then, the M4 contract was awarded to FN, and Colt sued and won, and now have exclusive rights to produce the M4 until 2007 or so.
HK makes a number of weapons for the US military beyond the Mk23. MP5s of course, and small quantities of several other models.
Benelli M4 Super 90 shotguns.
SIG Sauers are used by the Navy.
Glocks have been used in small numbers, but the US military won't widely issue Glocks due to the lack of a manual safety lever. The SIGs, being SA/DA, were apparently ruled to be "safe enough" for troops.
-Troy