KAC 600m sight.
I have two of them - one on my SR-25 and the other on my SR-15
As far as I am aware, they are the most comprehensive folding sight in such a small package, making them ideal to sit under a scope.
I've no idea what they are calibrated for, I have only dialed up by clicks for my given load. They are as every bit as good as a standard A2 sight and have used them effectively out to 500m.
I bought the Zahal flip ups for my XCR-M. I like them. Easy to adjust and good sight picture. They seem to be every bit as good as Magpuls.
I use Magpul Pro series on my RRA 308. They're great.