The rifle functions just fine, but I cannot stand how far the upper hangs over the lower. Fitment is the same with my Mega, Calico Jack and Honey Badger lowers, although the Honey Badger fit was slightly better. Any ideas on a better fitting lower, or should I stop being a pussy and control my OCD?

I'll admit that would bother me too. Not sure what you could do though.

Other side look the same? I'm assuming it does. You said it fits the same with your other uppers? Then it's the lower reciever, not the upper reciever. It would bother me as well. I'd trash that lower or call the manufacturer and ask if that is common for their stuff. Who knows, they may send you a replacement.

The lower actually sits flush with the upper on the other side. The fitment is the same with the other lower receivers I've tried.  

The Aero lower actually mates up well with my 16" BCM upper receiver... Could it be that the AAC upper is out of spec?

That's something strange, I'd call the manufacturer, they may be interested to see that. I've never seen one like that, it's for sure out of spec. Weird.

BTW, I'm no expert, I'm sure some older, wiser AR guys will come along soon and have a more definitive answer. I'd be curious to see if anyone has ever seen this before.

I vote to stop being a pussy and control your OCD

"THIS" is usually what I would say, but that right there would nag the shit out of me.

True...and even though it shouldn't happen, this is what can happen when you mix parts.

If only AAC sold stripped lowers... I feel like they would of had to base the design off of some manufacturers lower receiver. I can tell you it definitely wasn't Mega, Aero, Spikes or Sabre Defence.

I'll shoot them an email and see if they get back to me.

Having been issued weapons in the military that bad, it's not "wrong." They functioned, and milspec doesn't much care about "fit and finish." Once arsenal rebuilt they can range from sublime to just that bad.

Two different makers hundreds of miles apart, with a milspec +/- .015 in the finished dimensions, sure. Plenty of that is going to happen. The parts are drop forged, not fully CNC machined, and the total thickness across the flats thru the receiver is controlled by the amount of impact, hardening of the platter, alloy, temperature, and even the normal variances in the finishing.

Perfect example of just how far it can go. Not really everybody's cup of tea, but battle rifles and their makers aren't all about perfection in every mating seam.

There's nothing "wrong" with the lower, a vendor who is willing to let the other customers foot the expense of replacing it may do so. It's not coming out of their hide, no business gives anything away. It just pumps up the going street price on all the rest.

Luck of the draw, sell it if it's going to bother you. No guarantee the next one will mate perfectly, either. The vendor's selling complete rifles go to the effort to minimize it, but unless they are actually machining things, they don't have much control over it. They just sort them into compatible batches, which takes more labor, so the price goes up.

Ya get what ya pay for.

I have this problem occasionally. Even with Colt uppers on Colt lowers. I could take some pictures, but you already see the problem. My solution is to try a different upper until you find one that fits good. Fits front to back, side to side, and how well the feed ramps are cut and align with the barrel extension.  I have one Colt that is flush at the rear, and overhangs in the front. It's out of square (shoots straight).  You can look inside the upper and see if one wall thickness is thicker than the other and milled to one side or another. You can check your lower to see if it's cut to one side or another and match that to a receiver that is cut to the same side! Yes, I do that!  It would be easier to show you with pictures, but guys, it's early Sunday and im not playing photographer this morning. You get the idea that some are cut off center.  You have to find and match your lower to your upper to get the best fit. Not all parts work in all rifles. Have a nice day. -W

ETA: I get the receivers to line up as close as possible so the bolt carrier aligns correctly with the receiver extension. I always wondered, if the offset was enough, if the carrier would rub the buffer tube on one side or the other. I don't really know, I don't measure the offset to see where the error is. I just keep trying uppers until I find one that fits. -W

If that kinda thing bothers you (or anyone) then stick to matched billet upper and lower sets