The USMC ran the IAR program; it was apparently an attempt to get a reliable assault rifle under the disguise of a very light machinegun. So far they succeeded to buy at a few, but at least they now have a new type standardised and in service, so they may be able to get enough of them to replace their M16s.
But look at what they got; a thoroughly unspectacular design that was already MOTS if not COTS and merely modified somewhat with accessories. Basically the same weapon had been in service in Europe for three years previously AND the weapon is internally an early 1990's G36 with VISMOD outside.
The U.S. forces are still using the M240, an overweight 1950's design. Even the gold-plated (partially titanium) L version weighs 10.1 kg, while the Soviets/Russians have been running around with their equivalent (PKM) of 7.5 kg since 1969. Meanwhile, GI carried the 12.3 kg M240 for decades.
(The German Heer stuck with the almost entirely optics-incompatible and heavy MG3 for too long as well - it should have made use of HK21E in dismounted roles for decades). The Russians now have a 8.7 kg Pecheneg machine gun as PKM successor in service (and has so for more than a decade); it needs no spare barrels, so it's effectively even more lightweight than the PKM.
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