Ran a bunch of searches, but was not able to get the answer I needed:

I Just bought a complete Stag lower and the fit with my uppers is kinda funky.  The Stag lower seems to have thicker walls than my MEGA and Colt lowers.  Here is the deal:

When mated to LMT uppers and a Colt upper the Stag lower's wall thickness makes it stick out on the sides.  Hard to explain, but when you run your finger from the upper down to the lower over the Stag roll mark you feel a shelf where the halves meet.  In contrast my Colt and Mega lowers are smooth between the upper and lower, no over or under hang.

I have not tools to measure, but by eye it seems the walls of the Stag are thicker when looking down the mag well compared to the other lowers I have.

OK, so do I have something unusual?  I would prefer not to return it since it cost me FFL transfer fees to get it, but this seems out of spec.

manufacturing variability.

you just got one on the high side of the acceptable tolerance range.

as long as the uppers fit, i wouldn't worry at all.

Well, I just checked my Stag lower with early M16 upper kit.

I have 3 rifles. All have Colt uppers. One is a Stag lower, one a Superior Arms lower, third is a Colt lower. All three of mine fit rather well with little or no "overhang" at all. Certainly none worth getting excited about. Not a perfect fit if one were to put a high polish and "bluing" type finish and such on the receivers, but not noticable with the original black/gray matte finish.

Don't know what to tell you about yours.

All manufacturers may vary slightly in the specs. Since Colt holds the current TDP, it is the standard in regard to receiver wall dimensions, etc, notwithstanding FCG pin diameter and rear shelf web.

I have two Stag lowers and various manufacturer's uppers fit with varying degrees of overlap, looseness and dimensional perfection, as they will whenever you mix and match uppers and lowers by different manufacturers. Bottom line is it won't affect functionallity or accuracy, it is purely cosmetic and therefore not any source of concern.

If you demand perfection, then you should buy complete rifles that are hand fitted, or at least match receivers made by the same manufacturer, although this will not guarantee a seamless fit. If you demand functionality and reliablity, these issues are irrelevant.

In other words, I am of the opinion that you should not worry about these cosmetic issues, and use them as intended, as long as function is not impeded, which it should not be.

Your concern for overalap really isn't an issue.  It has nothing to do with function, and with the information you have provided, I can't see why the maker would even take it back.

If you posted a picture it might mean more, because with the info you are giving, it seems like a non-issue.

If it really bothered me, I'd send the lower to a dealer that has a lot of CMT uppers on hand and ask him to pick
you out one that fits more to your liking...

Or visit guns shows where dealers might have a nice selection of CMT uppers on hand...

Heck..try the guys a Stag they might even accomodate you..

Hope everything works out for you...

My Stag lower works great with my Bushmaster uppers. And is a tight, almost to tight, fit with a new CMT upper. But tight isn't bad. I don't have the shelf problem.

Thanks for the responses.  

I am not worried at all about cosmetic perfection.  Simply I have not seen this issue before and thought I would field the question to see if it was unusual and out of spec, but it sounds common.

After comparing my small collection of lowers (2 Mega, 1 Colt, 1 Stag) I noticed that the Colt and Stag have thicker receiver walls and the Mega's are quite thin in comparison.  

The Stag has a nice tight fit with all my uppers and fits best with my Colt upper dimensionally.  All mags fit fine in the Stag and everything seems good to go.

You learn something new every day.