Re: H&K MR556
+well finished
+solid system
+very, very good and well finished barrels. Some extreme accurate, most very.
-needs lighter trigger
-pretty heavy, and center mass in front. Slow to turn even compared to HBAR AR carbine
-PMAGs wont lock without minor mod
Also guys in A-stan (not american) suffered major mailfuncions, cant remember were they 308 or 556 tho. Fine sand+gunpowder residues sticked in gas piston. As result few rifles blew up, apparently due to breech which wasnt properly closed.
I heard rumour that HK fixed this issue already so shouldnt be problems with it.