The CMMG RDB uses a bolt that has angles on it which vary based on the caliber. Some think the extension on the barrel has corresponding angles on it. It is mentioned that way in their patent but to my knowledge, they never made them that way for production. I took the picture below of an early 9mm CMMG ‘Guard’ (what they initially called the RDB models and now call the ‘Banshee’ or ‘Dissent’ in 9mm). This extension had the dual feed ramps and looked like a typical M4 extension while later / current extensions have the single feed ramp but the lugs are still squared off like any other extension you see on a typical 556 AR15/M16.
Straight blowback barrels I modified for the CMMG RDB system or if you want to do ‘bolt action’ build.
I like using stainless as I don’t want to have to deal with refinishing and I may convert one to be used in an integrally ported SD build and at some point may drop the whole barrel in an ultrasonic with harsh chemicals and no concern of removing the finish if there is no finish to remove.
Note that the early CMMG Guard barrels just used what appeared to be typical M4 extensions with the dual feed ramps. This was before CMMG started re-selling the MEAN Endomag Pmag inserts which are dual-feed. Later they switched to a single feed ramp extension and they were only selling Glock fed lowers at that time. Then when they started re-selling the Endomags they still sell their barrels with the single feed ramp extension.
Note that if you are using Endomags, Exomags or CMMG 9mm mags, I think a standard M4 works fine.
Factory CMMG Barrels
This link shows some issues someone had: https://www.reddit.com/r/Cmmg/comments/1ffx279/brand_new_9mm_resolute_mkgs_dangerous/
In case the link becomes archived, I’ve saved the image that was there. This was a factory barrel shooting factory ammo.
Another bulged brass thread: https://www.reddit.com/r/Cmmg/comments/1kdu4eq/5_inch_rdb_build_buldging_cases/
This link also discusses 9mm accuracy with the CMMG RDB: https://www.ar15.com/forums/ar-15/Why-does-CMMG-RDB-accuracy-suck-/15-785228/
As discussed in that link, I do not believe there are any issues with the CMMG RDB system. I think there are/were some QC issues with the barrels as I have great accuracy using the modified barrels from other vendors. I also got decent accuracy out of a my second factory CMMG RDB barrel compared to the first one I got.