CMMG RDB Barrels

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.

So based on that knowledge, I had make me the barrel below by sending him a stainless straight blowback barrel, extension and 3 lug adapter.  Note that I had him thread the barrel to match the entire length of the 3 lug adapter to eliminate any free-bore condition where you have a bunch of build up after the threads.

Later, I wanted to try the CMMG RDB out in .45ACP and sent the components below to Rudy at

Today, I have my own lathe and have been converting my own barrels like below:

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.  I have a factory CMMG small frame Glock lower but mainly use Endomags.  Both work fine with the single feed factory barrels as well as the dual feed M4 style extensions so I have not bothered to alter the feed ramps from what you see in the picture above using a standard M4 extension.