I have nine CMMG-version endomags, two from CMMG and the remainder purchased as Pmags and converted.  I cannot insert the tan mags into my ADM lower if they are fully loaded.  The lips of the tan magazines clearly spread even with 28 rounds while the other 7 mags, 2 Gen 2's and 5 Gen 3's, insert easily.  If I download the tan mags to 28 rounds I can insert these mags with a bit of drag.  I don't know why the tan material would be weaker, but that certainly appears to be the case.

Step 1: Remove Endomag insert from magazine.

Step 2: Cut an addtional 1/8" from the limiter tab.

Step 3: Reinstall Endomag insert.

You should be good to go after that.