Birdsong. It’s what they use on the nuclear attack submarines. It isn’t just a coating, it’s a chemical reaction that changes the surface of the metal to be resistant to rust. Even after the color wears off the metal is still protected. It is also self lubricating.
If you are spending most of your time shooting past 600 yards with this optic, take the weight and get a 5-25x56 type of scope.
5-25x56 schmidt and bender pmii
5-27x56 zero comp
5-25x56 Tangent theta
Kahles is also a good option.
Also look at what reticles will fit your shooting style best...