This information is focused around the CMMG RDB system but some of the information is still relative to straight blowback setups. For those that do not know, straight blowback AR9 setups require a higher feed height than a gas operated AR15 as there is no bolt and carrier relationship, it is just a bolt. With the CMMG RDB system, it has a similar bolt and carrier relationship and the feed height is actually back to where it is with a gas operated AR15.
My picture below shows most of the popular options. Absent from the picture are the MEAN Endomag inserts and the CMMG 9mm mags..both of which have a built in feed ramp like the pictured Exomags. CMMG also makes a Sig mag lower. Hydra weaponry makes an MP5 magwell: https://www.hydraweaponry.com/product/hydra-9mm-smg-9c-modular-magazine-well-copy/ I know these have been modified to work with the CMMG RDB system as well after removing the fixed ejector from the lower but of course MP5 mags do not have LRBHO.
Glock mags
For Glock mags and the CMMG RDB system, the factory CMMG Glock lower is the best as it is obviously designed to work with their system. I like having ambi bolt catch and release and added a PDQ to mine. So added that to my lower as pictured below.
As seen in the picture at the top, the Glock mags being single feed typically don’t have a feed ramp in the lower and are also placed the closest to the chamber. This means the bolt doesn’t have to go back as far to strip a round off in the event the action may be dirty or if you encounter a slightly underpowered round. I also think it requires the least amount of energy to strip rounds off a Glock mag than any of the double feed options.
There are also several mag extensions for the Glock. The largest I have seen being the +30 Taylor Freelance Goliath for 61+1 rounds. Stick mags are easier to deal with when running around on a match vs a bulky drum.
While I personally think Glock mags look ugly in a PCC, I cannot deny that they do work great and feed all bullet types with no issues. They can even keep up with the high cyclic rate of this post sample Maxim 9 picture below that was clocked at 1500RPM.
I think one valid concern is that since the outer body is polymer that it can get chewed up with the metal mag catch that is used in PCC’s. I haven’t encountered that yet myself.
Colt mags
Often praised for the way they look. I used to use factory Colt and Metalform (who was the OEM for Colt) 20+ years ago on a regular basis in SMG matches. They were always notorious for ‘volcano’ing’ when you drop them. Where rounds would spew out. I think they would sometimes let go of rounds in the action as well. We put up with them back then as this was also doing the 1994 – 2004 high cap mag ban so you used whatever you could get. In this case grandfathered Colt mags. Original Colt mags had metal followers, later they came with plastic followers but the Metalform to always use the metal followers from what I have seen. I think the metal ones have a more robust LRBHO. I have more detailed info on using Colt mags with the CMMG RDB system here: https://c3junkie.com/?page_id=374
Scorpion Mags
I think Scorpion mags look the best with the curved MP5 look with LRBHO. With several sources making them, they are not expensive and work well……
However, there is currently no lower out of the box Scorpion lower that works with the CMMG system. I went through a lot of work to try to get re-branded NFA Scorpion lower to work as documented here: CMMG RDB with unmodified Scorpion Mags I have finally having it running well but only with RN bullets.
The MEAN and CMMG mag options are the most convenient for standard AR15 owners since no mag block to deal with and you can simply swap uppers to change back and forth from 556 or 300BLK to 9mm.
MEAN Arms Endomags
These are inserts for Pmags and from MEAN has told me the most reliable mags are the Gen2 Pmags withOUT the windows. Available with the ejector built into the mag for straight blowback users which I do NOT recommend since you have to remember to leave the mag in the gun to clear the action. Then there is the ejectorless version for use with uppers the CMMG RDB system, MEAN bearing delay upper and the Angstadt MDP9. CMMG was licensing this insert / mag until they came out with their own mag.
MEAN Arms Exomags
Complete mags instead of an insert with a max capacity of 40 rounds.
CMMG 9mm Mag
It is my understanding that this is not licensed from MEAN and CMMG’s own mag. 33 round max capacity.