The 556 I am not considering for the following reasons:
1) if I get a 516 or a LWRC as i already own copies of those it will simplify my spare parts situation and since with piston I am a bit out of the mainsstream I am determined to do that
2) I once owned a 556 and sold it within a week w/o firing as I despite its strong features ( adj gas, CHF barrel foldable stock) reason being the handguards were terrible
moving sqeeking squirming with every grip..etc, replacing them with a SIG only rail would have been an etxra 200 bucks
3) I didn't like the balance since I am a 20 yrs soldier and it felt too weird to me.
But i thank you for bringing up an interesting and much overlooked rifle