I was at a 300 yard range today and was shooting some of my other guns but I brought my x39 AR along because it is so much fun and cheap to shoot.
I have a 3x scope on my x39 AR and I'd never shot it at more than 100 yards. My scope is an Amazon cheapie, but it has a graduated reticle with cross hairs for 0-300, 400, 500, and 600.
I found that if you use the 500 reticle, you are almost perfectly "on" at 200 yards with 7.62x39. I'm sure this would be the same even with a different scope, as long as it was set up for a 5.56 rifle.
I didn't try the 300. I wish now I had. But at least I can hit 200 yard shots with ease now.
I have several hundred rounds through this Radical Firearms upper, AIM bolt and carrier, and using ASC mags. Still running like a top, no mods except removing the O-ring from under the extractor.
Fun.