My Mk18 runs flawless. And I've beat the living $hit out of that thing. Running it time and time again, without even cleaning it. I think she's down to only one of the gas rings left on the BCG. The others have twisted off over the "many" rounds put through her. And she just keeps running. Now that has absolutely nothing to do with the rail system that's attached to it. But the Daniels RIS II that the Mk18 runs, is every bit as good, if not tighter, than any other quad rail made for the AR-15 platform. Even compared to Geissele rails, that you can literally BEAT people with, if the rifle ever goes click instead of bang.
The RIS rail for the large frame AR is pretty much a unicorn at this point. Made from Unobtanium as it were. They can't be found. Anybody who has one......isn't letting go of it. Period. Hell.....I'm still looking for one too.
But Daniels's being Daniels's. I'd be willing to buy into the RIS III rail. I would guess, right out of the gate, that the platform is gonna be solid.