THIS. In addition to what is mentioned in this post, the tabs prevent the gas tube from being pinched, should the rail come loose and rotate with great force. All of the TOP manufacturers, military contractors or not, have engineered some form of anti-rotation into their designs. Noveske use a pin, DD uses machined tabs on the bolt up collar, Geissele uses tabs on the rail with set screws, BCM uses a totally separate steel insert. All of these companies would not waste time and money if they didn't see a need for the system.
You are correct about some of my observations. If ATRS loves their sharp mag well due to aesthetics, then so be it. I don't, and I'd prefer to have something more friendly to grab. This can for sure fall under the "to each, their own" category. However, there should be no gap when installing the front take down pin. The shoulder of the pin should make contact with the side of the lower when fully closed. I see that as an error. Having the cutout for the charging handle directly below where the tabs on the handle will ride when the bolt is locked to the rear, I see as a poor design choice. If the normal cutout in the upper had to be relocated forward due to dimensional changes, why not remove it all together and have the handle pull directly out the rear? Is there a requirement for the cutout?
With exception of the two things I mentioned above, I think ATRS made it very well...of course some things I would have done differently, but that is their call. I'm not a firearms manufacturer. I actually tried REALLY hard to not be combative in the initial post! I guess I'm just an abrasive person. I didn't mean for the post to come off very negative at all, but instead, just wanted to share my opinions/observations. This post is exactly what I experienced when building my rifle with their receiver sets. I really do like the rifle and I GREATLY appreciate ATRS for bringing it to market. People on this forum get all triggered when the read something even remotely critical, especially when they've invested their own money into the product. I've seen it here a thousand times over the years. In any case, I'm curious as to what specifically you agree with? Do you own one of these things? I've sent ATRS an email and linked the post, but no reply yet...probably too busy cranking out receivers which is good. No one has replied about my charging handle observation.