That should work just fine. I have used optics on all kinds of AR configurations, including the old style 4x on my 11.5"/5.5" Commando, and you're probably not gonna get sub MOA clover leafs, but that doesn't mean it won't be effective. I assume you plan to use it for Varmints, so effective kill range won't be much of an issue. It will help you pick out small critters hiding in the brush. So if you just have one laying around, give it a shot, see if you like it. It'll be way less tactical than a dot sight, so you'll have to turn in your Ranch-Ninja badge.
ETA: I obviously can't finish reading a sentance before I start posting. If you have a lot of 500 yard shots, it'll probably be more a matter of your rifle being underpowered at that range (Like HIWATHL said). If that is your "maximum engagement range", then it is up to your judgement to make the shot you are confident with.