[Boyso]

IMHO, it's more for the looks than functionnality (Just like airsoft, almost).

The farther the sight is, the smaller the crosshair is going to be. On a M4, mine is about over the magwell.

2, it is not parallax-fixed, meaning that if you are not exactly in the same stance (let's say, cheek on the stock, you're going to be off-center.

So get BUIS if you really want to double-check.

Elen, knock on the side lightly, sometimes, the mechanism is stuck.