No chrome lining from H&K USA.
German Export law forbids whole rifle export for non governmental usage.
H&K USA puys H&K German and USA parts to gether to sell the MR556.
Nefarious Arms is making a run of chrome lined barrels.
MR223 (Euro/Canada version) - available w/ A5 features/ambi controls in the "MR223A3" variants. MR223 variants have chrome-lined barrels
MR556 (US export) - only available as the "legacy" A1 variant. No chrome lining as barrels come in "unfinished" and are completed here. They are chrome moly or whatever the HK Aubert-Duval specific alloy is.
German Export and US Import laws are the cause of the problems.
MR556 varies from legacy 416s in the carrier has a little bit of material removed to trip an auto sear. There is a tab on the barrel extension to not allow 416 carrier's to be used as the 416 carrier does not have the cut recess for the tab. The barrel is not chrome-lined and it has a heavier profile than the "socom" profile 416's. MR's also have special push-pins that lock into place and take up receiver slop (but most change them out for milspec or aftermarket). Current MR556's come with the HKey mod rail that's unique to commercial models rather than the legacy quadrail.