CMMG currently charges $975 for either a 5″ or 8″ FE Banshee upper. Or $1050 for a 16″ Resolute upper. https://cmmg.com/upper-receivers
IMHO, building out your own upper is the best option. It is not only the most cost effective option but also allows you to pick and choose your own components that you like the most (rail, charging handle etc).
The heart of the RDB system is the proprietary bolt and carrier. Their is nothing special about the barrels other than them using an extension. The FE version also relies on a fixed ejector that is bolted to the upper. CMMG sells their proprietary BCG and a FE equipped upper for $425 retail: https://cmmg.com/fixed-ejector-retrofit-kit-9mm
Then you need a barrel. If you want a ported barrel then the ones from BRT are popular: https://www.broadrivertactical.com/products/brt-6612-908c4ff7-078c-4fba-a0a2-08e477b01838
For non-ported barrels you can buy them CMMG as well but be warned that QC has been spotty on them. This is discussed with several threads referenced here: https://c3junkie.com/?page_id=1882. I sell 416R stainless barrels in 5″, 8″, 10.5″ and 16″ threaded 1/2-28″ for less than CMMG. These also have the same single feed extension for feeding all mag types. https://c3junkie.com/?product_cat=cmmg-rdb-compatible-barrels-also-for-bolt-action-builds
Some people have asked me about stainless durability…this is pistol caliber. I don’t think you can wear out a 9mm stainless SMG barrel. I have one with over 50K on it and it still holds tight groups.
So for $625 you can have the main components specific to this build (barrel, BCG and upper). Then source your own handguard and charging handle. So under $700 if you want to get a cheap rail and CH. Note that unlike straight blowback 9mm AR builds, you can ‘pretty much’ use the same lower for this 9mm RDB build that you would use for 5.56 / 300 BLK etc…
While you can see that the carrier has a hole to accept a weight, it doesn’t come with a weight and IMHO, the carrier is already kind of too heavy for the 55 degree bolt that it comes with.
Here is a picture showing the underside of the FE upper. The auto sear relief is also used for the Arizona Regulator FRT.
Doing a CMMG RDB FE gets you these benefits.
1. IMHO, it is the only closed bolt platform that can run at around 600RPM with low reciprocating mass of around 16 ounces or less (my latest configuration is 12.2 oz total). This is of interest to those with a RR / RDIAS / RLL / SS / FRT. If you want faster, that is easy too. Very tunable. It is also drop in for a RR / RDIAS / SS / FRT. There is no weight you need to trim or special firing pin springs you need to get.
2. Being delayed blowback, OOBD are just about impossible as the bolt must rotate inside the extension for the right amount of FP protrusion to set off the primer.
3. Being delayed blowback, it is also a better suppressor host than straight blowback.
4. While the BCG / upper are specific to this build, all other parts are standard AR so no special parts that only get used for 9mm. (this is assuming you are going to use Endomags / Exomags / CMMG mags). This means leverage all the various ambi devices etc.
5. Want to run Colt mags or Glock mags or Scorpion or MP5? I go over some of those details here: 9mm AR15 / M16 Mag Options