As another fanboy....who doesn't have one.... They are really pretty nice guns, and as expensive as they are, HK is not making a mint on them. There is apparently a bunch of typical HK uber anal stuff in them that makes them expensive. I will likely get a MR762 at some point as I am looking to build a SD designated marksman version. This is the same role where the MR556 also makes sense. They are heavy pigs, but they make a good designated marksman rifle and they work really well with the OSS cans.
For a lightwieght build, Centurion Arms CMR rail on a 6920 is a good route. Monty at Centurion just built me a gun with an A1 barrel in a CMR rail and it is simply wonderful and shoots great while super light. Or you could go full nyeti and get an AUG.....