[asshanson]

If you are going to be doing a lot of various ranges, something like a 4-9 or 4-12 power would be good. The 4x does not mean good for 400 yards specifically.

You also need to choose the diameter, like 40mm or 50mm. Bigger diameter is going to let more light in, but also going to bring up the center away from the rifle making it harder to use. But it looks like your rifle has an adjustable cheek riser so that is less of an issue. Better optics also let more light in and have better clarity, but of course you pay for it.

Since it's a combat assault rifle, I might suggest a nice red dot sight. No zoom, but much quicker target acquisition and still a lot better than iron sights. With my dad's M1A Scout and red dot sight I had 2" groups at 100 yds first time I shot it (with mil surplus ammo). The dot itself is only 1 moa so it doesn't cover your target, and has adjustable brightness and an infrared night mode if you feel inclined to wear night vision goggles. Plus they don't weigh anything compared to a scope.