Anything but the poorly designed gas tube system they have now.
Delta Force operators are using the HK416.
75TH Ranger Regiment is also using the HK416 along with the FN-SCAR. Tried and tested. If they work for the elite forces why not outfit the regular line units with them. It's all about reliability.
Not sure where your getting this information concerning the H&K 416 with any SMU units. The SCAR is all but done and has been handed back in to SOCOM. It is no longer goes down range any of our SOF units, both the heavy and the light versions. You can find these facts regarding the SCAR on just about any military website concering weapons development.
I have it on good faith from members of the SF and SOF community that many unit purchase have been made of the LWRC piston system rifles, with some Teams. The H&K 416 was even pulled from 1st BATT 1st SFG (A) in Japan. :fie:
Just because this article mentions that soldiers where killed in Minat does not mean that the M4 5.56 rilfe was the culprit of their deaths on the battlefield. More information regarding those deaths would be needed to assess the reasons and what weapons or explosives they died from. Snipers have been known to set up a sniper hide as close as 400 yards to entrenched positions. At that range I can use the iron sights and hit COM shots and draw in medics and take head shots pretty easily. Take one out of the fight and let the others come to his aid and pick off everyone of those also with iron sights, at that range.
This seems to come up every year since the GWOT started with the M4 rifle. Maintain our rifle, keep extra parts kits with you, lube it properly and train the conventional white side of the military how to actually employ the weapon and it will get the job done without having to train already mediocre soldiers on a whole new unproven weapon system.
Then there is thw subject of ammunition weight during dismounted patrols into the high mountainous territory. Try carrying 10 mags of .308 Nato versus 12 mags of 5.56. You'll see the difference.
I'm no expert here but a nice choice would have been the 6.8 SPC, either Direct gas or piston system. Regardless of it's pitfalls, it beats the 5.56 any day of the week.
Here is one short blurb explaining the recall of the SCAR-16:
http://www.thefirearmblog.com/blog/2010/06/28/socom-abandons-fn-scar-mk-16/
Just my .02