Had the M4 OPS and ended up trading for an Aimpoint. They're made by Walther in Germany. They look gorgeous and have very accurate barrels, and you can adjust the recoil spring tension, but that's about where the list of pros end. The bolt / carrier is enclosed in a sort of metal case inside the upper receiver that must be disassembled to clean. They supposedly said that these rifles didn't need to be completely disassembled to clean but that was not my experience. It was a jam-o-matic once I would get to around ~500 rounds between cleaning. I would also like to note complete disassembly voids the manufacturers warranty.
That upper metal case I was talking about? Yeah, that also holds the barrel in place.
This rifle uses an INTERNAL barrel- what you see on the outside is basically a shroud. It very much reminds me of an intricate airsoft gun. Overly complicated and a bitch to keep running. Needless to say, if you WERE to disassemble the rifle for cleaning, you would certainly need to rezero your sights.
The FCG is also enclosed in a metal case but I never messed with the lower.