You can have a look inside of your magwell, the marking should tell you which part of the mag is dragging. If its the issue of the rivet, make sure it has enough meat for you to trim.
Pmag gen2 is known to be a bit fatter, and M3 should be slimmer.
Most of the non-us made rifle using stanag 4179 mag will have tighter magwell, since they are designed for aluminum stanag mags. If you still have issue, try aluminum or stainless mags from reputable company: colt, elander, FN, ASC, surfire, ext
And yes, there are unknow brand of metal mags which does not meet stanag 4179 standard.