They will work
Will it work on an MR556 or vintage 416 upper?
Own: HK91, SR9, SP5k, G36k, HK416, P30L, 2x P8, 2x P8A1, USP9
Future: Done, until HK comes out with something new & nifty. Might start building an FNH collection.
Are we sure?
I'm specifically concerned about the sling-loop on the old-school gas block.
I had one that came with an old 14.5 upper. It would fit
Just so ya know, the tongue and groove on the top mated just fine, but the tongue portion of the rail sits lower. Thus you won't have that flush fit on top (not that it matters at all functionally).
Just the tongue. Rail lines up fine. My fit was like some of the other rails as of late...a little slop but fine after tightening the cross-bolt...I've been spoiled by the super tight Geissele's.
I figured this is HKPro...home of the "OCD9". Best you find out now.