I've been thinking about the issues with 3dp uppers on 3dp lowers during a project and remembered that early in AR development, before the familiar takedown pins they tried using a dovetail/flange system where the upper would slide forward off the lower.
My thought is that this design could be used to make a 3dp assembly a lot more resilient than relying on a pair of pin bosses. Done the way I'm imagining the lower could be kept compatible with standard uppers, they simply wouldn't engage the female feature and get pinned in place as usual. But the matching upper would have vastly more contact area, rely on one or both pin locations simply to retain the upper from sliding forward, and allow for another surface for the upper to register against besides the buffer tower, the end of the dovetail on either side.
The project I'm working on btw is an AR pattern semi auto shotgun that will use an AR10 style lower and feed from modified .308 mags. I was flipping between giving the traditional reciever assembly a go, and just designing a monolith reciever and sidestepping the attachment issue altogether.
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