It's normal, optics give a darker scene, like in day and like in real life. Without night-optics or night-vision impossible to have enough light through the optic.

Do you have any reference to support your statement?

Have you ever use binoculars?

The collimator is self illuminated and you can adjust the brightness of it for NVG use.

Ok what you say is true for the ACOG but I found nothing on the ARCO.

http://en.m.wikipedia.org/wiki/Advanced_Combat_Optical_Gunsight

If ACOG do that sure enough any other us army candidate would do the same or better.

Maybe you must post a tread on the forum for this.

Yupp, in fact it looks a bit like the ARCO actually ist illuminated ingame, but only faintly so, as the reticle is visible when aiming at a dark spot but almost invisible in somewhat brighter areas, like in your screenshot. Maybe thats on purpose, seeing how the aimpoint on top of it is properly lit, but then i don't see the point (no pun intended).

Anyway, I couldn't find any real life information regarding this scope (or what it might be based on) but all comparable ones are illuminated, as is pretty much standard for this kind of optics, so untill i get a decent explanation why it should be like that I'll call it a bug.