Forgive me for being forward but why are you buying an AR when your intention is to shoot 400 yards in the future? The Sport is a fine rifle but I don't think it's going to do all that well at 400 yards without some pretty serious upgrades. And a 3X scope isn't really something intended to get you out that far either. It certainly can be done but most people pick a more powerful scope for longer distances like that. It's actually bad to go with a scope that is too powerful because of mirage issues but at 400 yards you're just starting to get into the distance where that becomes a factor.

I've shot AR's built for accurate, long range shooting at my gun club on the 400 yard range. I shot an AR-10 with a heavy barrel for example. The larger bullet from an AR-10 will stay on course better because of the weight it isn't affected by the wind so much. I also have a bolt action .223 that's built for shooting that distance and farther. I've shot several AR's out to 400 yards actually. I've never seen one come close to the accuracy of the bolt action rifle I have. That bolt action rifle I have is a particularly accurate rifle (Savage 12 LRPV with the target action) and I have a Weaver 36X scope mounted on it. For a long time I had an 18X scope on it. That rifle shoots 5" groups at 400 yards. I've never seen an AR shoot better than maybe 18" at that distance. Maybe I just didn't get to shoot a good example. I know some AR's do shoot very accurate but they are purpose built examples and not entry level rifles with 3X optics.

I don't mean to offend you. I just wonder if you've thought about these things because it isn't going to be that easy to shoot well at that distance with that rifle IMO. I bought my Sport as a 100-200 yard maximum range rifle. Most of my shooting will be inside 100 yards. Mine is pretty accurate for a rifle of this type. But it won't come close to what a dedicated long range rifle will do. I think you'll find it's pretty darn hard to hit anything smaller than a 18" target at 400 yards with the Sport and that's probably being optimistic.

You say you want to buy a better AR down the road. I believe I would make that rifle your long range AR. If I wanted a 400 yard capable rifle now I'd be getting a bolt action. From what I understand you haven't bought an AR yet. There's plenty to like about them. But shooting 400 yards with what is really more M4 than AR (judging by barrel length) is pretty ambitious.