I picked up a CMMG Mk9 w/ 16" barrel at a recent gun show.  I bought some ASC 32 round mags for it and they seat and lock into the lower just fine, but they rub against the insides of the upper making them difficult to insert.  On the rear edge of the feed lips, all of the finish has been rubbed off the mags after just a few insertions and I'm getting wear spots on the inside of the upper where the contact is occurring.  With a full mag, between this friction and the compressed spring, it's almost impossible to seat the mag with the bolt closed.

I've only had a chance to run one mag worth of ammo so far, and it ran flawlessly.

Has anyone with a Mk9 dealt with this before?

I have some but haven't had time to test.  I'll give it a go tomorrow.

ETA - Nope, no problems.  I forgot to try and load a mag with a closed bolt.

I have a CCMG lower and DPMS slab side upper on my 9mm build
mine does the same thing
makes removing the mags hard
I just dremmel sanded the area inside the upper that catches the mag lips
very little was needed
now they insert and drop freely

I have the CMMG MK9 PDW and I use ASC mags exclusively (no issues btw).

I think ALL AR9s in Colt pattern are nearly impossible to seat a loaded mag on a closed bolt - it's not a CMMG or ASC issue.

I too had to do this to my Vltor upper. It was approximately .010 on each side.
And I also can't hardly insert a loaded mag when the bolt is closed.
Thanks for that bit of info.

I have the same problem.  I bought 10 ASC 25 round 6.8 mags.  Some rub or even stick in the lower, some rub in the upper.  ASC has ignored me.  The ones that I've been able to grind down to fit don't feed reliably.

My new rule is:  When buying an unproven mag, buy only two to test them out so I won't get ripped off quite as bad.

ETA I ground down the mags instead of the upper because the mags are cheaper and easier to replace.  Did I mention that ASC didn't care that their mags were poorly made?