I put together a CMMG RDB AR9 build using the 5" RDB Barrel and BCG combo and I elected to use milspec upper and lower and go with the ARC9 conversions. After my first range trip I had zero issues with the RDB build in terms of firing and reliability (360 rounds from 6 brands of brass cased range ammo between 115-147gr). However I did have some questionable observations regarding the ARC9 magazines.

I ordered my ARC9s as a pack of 3 from MidwayUSA on 4/30 and today was the first time I used them and fired 120 rounds through each. My main gripe is that it seems to be impossible for me to seat these things into a closed bolt and since the BCG is CMMG's RDB, and these ARC9s were designed to work with it, something seems off? I noted that amphibian has posted pics comparing older vs current gen ARC9s and yet the followers on mine doesnt seem to match either? Also I did cut my round limiter tab in accordance to amphibian's recommendations, and I can confirm that the limiter is not bottoming out.

Secondly, while examining the ARC9 mags, I noticed that the top of the follower that is molded into the shape of a 9mm round has some gouges on it, all 3 of my ARC9s are gouged in the same spot (Pic linked below). I'm guessing this is due to the RDB BCG being released from the locked back position while the mag is empty? Is this normal?

Thanks in advance for any input. I will say the build did not disappoint me but I would certainly like to at least iron out the issue with not being able to seat the fully loaded magazine into a closed bolt.

I get the same with my EP9 RDB Build. Figured it would just be apart of it, but no issues thus far.

I have 9 of them I have been using.  I ordered most of them direct from MEAN when they came out.  I also have (2) 3 packs I got from CMMG at different times.  The first 3 pack from CMMG is identical to what I got from MEAN.  The 2nd 3 pack from CMMG that I just got last week has those bullet shaped followers you have shown.  
I haven't tried any of these and they are still sealed in the package.
I have had zero issues with the 'older' ones.
So I guess there are at least 3 different generations of these now...gotta update my site again.

Thanks for chiming in. I was very happy with the performance of the build, the magazines not seating when fully loaded was annoying but doesn't seem unfixable. I'll reach out to CMMG and see what the say about it.

So I had a call with CMMG this morning. Andrew the rep I spoke with suggested that my magazine catch or lower may not be in spec, thus causing issues with seating a fully loaded 30rd mag, and the gouges from the BCG. Thinking that my lower is pretty reliably in spec, I scrutinized the magazine catch and was able to determine that the mag catch could be rotated one rotation tighter to allow the magazine to seat more reliably, to the extent that the issue with not seating fully loaded mags seems to be cured (yay).

I've also marked up the gouged follower with a silver sharpie to see if the gouges get any worse, but I have a feeling the BCG has already done what it would do by now, luckily the gouges are purely cosmetic. Hopefully this info is helpful to anyone else that may find it hard to seat a fully loaded mag.

I would either load to 29 rounds or load on an open bolt.  I suspect if you extract that first round loaded from a closed bolt you'll find it pretty nicked up.

I just tested 6 cmmg mags for an up coming pcc match. I feel the hard seating of the mag is due to the raised portion feed ramp that is built into the mag guts hitting the bottom of the bolt. I sanded down the tops until they were even with the feed lips of the magpul mag and that helped alot. I also sanded the bottom of the follower to give the bullet stack a little more room to compress. Half were gen 2 the other half gen 3. No difference between the two as far as seating difficulty.

I read the CMMG description saying something to the effect of the upper needing an enlarged ejection port. From the pics on their website, it just looks like a mil spec upper. Have you had any ejection issues? Maybe they're just referring to enlarged vs. the Colt-style 9am upper with gas deflector and shorter port door?

This has been discussed several times.  I use the Aero XL (previously called the 458 uppers) to be safe but many say that they use standard uppers for 9mm with no issues.
You can see in the picture below how the port has been clearanced forward of the port detent groove.

I be one of them.  Perhaps it is a thing with RDB, but my blowback works just fine with a standard ejection port.

Ah, okay. That's a pretty subtle difference. Thank you for the clarification!