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.
That bit about the Corps going after the HK416 in an attempt to get leverage to replace M4s/M16A4s has been passed around the internet so much that people asssume it is true. We went after the HK rifle to gain a true Automatic Rifle to, and the acquisitions folks actually did a half decent job for once.

We are not trying to disguise it as a LMG. Everyone driving the employment and doctrine for the IAR is very clear what it is, and isn't. The gun rags and fanboy sites typically get the information wrong.