Just wanted to share this build since it's not a common setup. Since CMMG doesn't offer it, I decided to attempted to build it myself.

...CMMG RDB (radial delayed blowback) with a AR9 lower running on CZ Scorpion mags (kinda, must use manticore mags)

The lower required a few minor modifications to clear the CMMG bolt/carrier.

Put a few hundred rounds through it so far...a couple of stove pipes with first few mags. Ran 100% so far on following couple hundred rounds. Want to run it some more before I decide to keep and sbr it.

I call it the poor man's MPX

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nice build, this is the next one I am building (backorder an a really cheap RDB 5") with another mod to get me one step closer to MCX at home.

Nice build!

Very nice. I'm a big fan of PCC's.

This is brilliant, what AR9s should have been from the beginning. Well done.

What is it that makes the Manticore mags necessary?

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The manicore feed lips are metal with smaller profile that clear the bolt and bolt lugs of the CMMG RDB system without modification to the mags.

PSA mags can be modified to work as well (see pics)

Since all other Scorpion mags have plastic feed lips, shaving them down will most likely have structural issues with cracking and flexing when mag is loaded up.

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I would love to hear what sort of mods the lower needs. I did not realize that there was a AR lower that takes Scorp mags, that itself is pretty cool too!!

Def interested to hear how the mags hold up since CZ had issues with feed lips cracking.

NICE BUILD!!

I did basically the same build the I used PSA mags ( steel feed lips) and had great results. I SBRed mine it runs flawlessly I just need to work on the lrbho.

Very nice! Looks like you are running a millspec upper receiver...did you have to open up the channel for the magazines to fit?

I did clip a couple coils off the bolt catch spring to aid the weak lrbh, has been 100% so far on my last few range visits.

For folks who are interested, I'll post some photos of my modifications when I get a chance to snap some photos.

The only mods the lower requires is:

- removal of ejector (only held on with a couple screws)

- the outter wall of the feed ramp channel needs to be ground down low enough to clear the bolt carrier (used a belt sander). Spent about 30-40min slowly grinding and test fitting. Could probably knock it out in 20min or less doing it again.

Note:I did notice that the copper of the bullets got chewed up quite a bit due to the feed ramp being a touch higher than it should to properly feed into the chamber. I ended up using a round file to bring the ramp down a touch for better feeding alignment with the bore.

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Yes it is a milspec upper I clearenced ( read hacked with a dremel) for the mag I struggled with modifying the mag catch for magazine ride height to clearance the bolt and then to much clearance and it would not pick up a round. I took a few coils off to try and get the lrbho to work but the mag rides a bit low it will hold if I push up hard on the bottom of the mag but then the bolt drags. It feeds and runs perfectly iit just does not lock back. I’ll mess with the rod and see if I can get it to reach deeper into the mag well. I am running a kyshot hydro buffer and Flat wire spring it is super smooth. I have gotten a ton of advice from a few members here it has been a fun build.

Interesting, I didn't touch the latching shelf on my PSA mags, all I did was shave down the plastic to expose the metal feed lips and grounded the metal lips ever so lightly to ensure no contact with the bolt carrier.

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Here is a few pics of a unmodified and modded receiver next to each other...

The re-profiled feed ramp is a bit ugly but plan to clean it up so the both channels match better and will reblue it after it gets a good polishing.



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Few more pics of the PSA magazine.

Again, only modified the feedlips. I did NOT have to modify the shelf for the mag catch.

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