They are saying no to it at the moment. Going to make their own they say. No more 5.56 or 7.62 and no more brass. Looks like it will be 6mm'ish plastic cased ammo.
The HK 416 is the Mercedes Benz of AR technology. They run very cool with the piston system under sustained fire conditions compared to a direct gas M16/M4. I handled and fired one once several years ago and was impressed but the truth is, I've never had a problem with the direct impingement system as originally designed by Gene Stoner. There are less moving parts and the weapons will run just fine with proper cleaning, care and lubrication. There is a bit of a stink about what the USMC M27 will cost the taxpayers too. I think to the tune of $1100-$1200 per unit.
Not really cost effective when you consider the losses anticipated during the first hour of an assault. Remember guys, the first rule of the Gum't when purchasing isn't best...but cheapest... I'd be surprised if the sun doesn't set on this horizon.
The Corps seems fixed on the M27 guys and with US licensed manufactue ( that I am sure our Uncle Sam will insist on) they should be able to bring the cost down with expanding orders.
With its HBAR profile, a shift to a different intermediate caliber wouldn't be an issue, providing it was conventional brass and not some plastic/caseless hybrid...
To be honest, I personally don't see much of a point. The reality is the rifle is just a very small part of what actually is used in combat for kills, and practically I don't think you will notice much of a difference between the M27 and a Vietnam era M16 (condition being equal). You need a serious advancement in rifle technology to gain any real advantage in infantry weapons and we haven't really seen that since roughly the 1950s/60s.