120mm,

Uboat is right on the money, I did something very similar to my rig (because he is right those sights are crap) and was able to acquire that sight lead with only a little practice... I think his Jpoint suggestion is an excellent one and would have definitely added it if I could. In the right hands this setup makes a shooter surgical at almost any range.

One last thing about the ACOG that can not be said enough, I don't know why I didn't say it earlier, they are RUGGED. I put mine through hell and always had this mental itch to check my zero but the dope was always good...they are made simply and solidly.