So I have placed my order for a DPMS LR-308 AR-10 rifle. My question is what scope to put on it. The gun will be used at the range 99% of the time at 100 and 200 yds. The other 1% is for when the zombies show up. My eye sight is not the best so I will need at least 16x power. My budget is $400. What would you all recommend and why. Thanks all in advance..