So, here goes. As you know, the iron sights on the M4S fold down. The front sight folds down and is locked in place by a button mechanism, very nice. However, the rear sight is held down with a U-Pin, which has tension via a spring, which is located inside the rear sight assembly. The spring has a little hook at the end (left side) that goes into a hole in the U-Pin. But the hook is tiny, maybe 1mm long, while the hole in the U-Pin is 3 - 3.5mm (don't quote me on these measurements but you get the point). This obviously makes it easy for the U-Pin to slide out of place and therefore the rear sight cannot properly be held down.
I asked KWA to send me a replacement spring in hopes that maybe the one on the gun is defective or damaged. Nope, they look exactly the same. I tried my best to get it to work but something isn't right as there's no conceivable way the U-Pin will be held in place with that little spring in normal game use.
That being said, I do appreciate KWA's responsiveness to sending me a replacement spring (and U-Pin), but I think this might be a design flaw.
Correct me if I'm wrong.