I’ve tried two different RDB uppers and both are inaccurate across multiple loads.  Each spring I retry getting the RDBs to group and each year I’m disappointed.

Any ideas?

Here’s this year’s 15 yard bench rest comparative test.  147gr with BE-86 but the load has never mattered much

Top down:  SP5, Lage M11, RDB, RDB, Ancient Olympic arms 9mm

What type of muzzle device?

This was with a suppressor.  I’ve tried everything from none, different cans, and also a comp/flash hider.  It comes out relatively the same across these guns regardless of muzzle device

I had a very early CMMG 9mm RDB barrel that had a standard M4 extension that had terrible accuracy.  The later barrels have the single feed ramp extension.  That one shot a lot better.  I don't know if they outsource their barrels so who knows or maybe they change blank suppliers periodically as well.

I typically run my reloads almost 100% of the time.  
I have some pictures of some groups I did several years ago and the tightest were some factory Winchester TCMC rounds.  My reloads using Berry's 147Gr were all over the place with Titegroup but were much better with VV320.  I was using a lot of the RMR 147GR JRN at the time w/ 3.7Gr Titegroup and getting really nice groups on that second barrel.  I later went to a custom SS barrel and that barrel has been great too.

I guess I’m still screwed 🤣

I hate to throw more money at these.  Especially custom barrel money.  

I really want to like these uppers but it’s just not happening.  The M16 controls, no mag blocks, and operation would sure be nice for competition.

I’m looking forward to seeing what your new design/style upper is.

Here is a picture I took back in 2019


This is the same exact barrel that I have in my GB ad right now.  1037533564

I'll find out if it is indicative of the platform or the cartridge as I am putting together a 45 ACP version RDB right now.

I thought you already had a 9mm CMMG RDB?  If so do / did you have accuracy issues with that?
I also have the CMMG RDB in .40SW and .45ACP.  I've run my .40SW in matches and accuracy is fine with that one.
I have not run my .45ACP as much.  I just made sure it ran in full auto unsuppressed and suppressed and haven't shot it much to really know.

Note that the RMR projectiles have a larger than normal ogive and they state on their website about having issues with some barrels that have short leade's.  That is the case with the CMMG RDB barrels and the RMR bullets as you have to mortar the gun sometimes to clear a live round unless you go 'buff' out the leades some.  
That said those RMR bullets are accurate for me.

Regarding the Berry's I stopped using them a long time ago as I ran into a few batches that were .354 and being plated, are supposed to be .356 and I paid extra for their 'double struck' bullets which are supposed to be even more properly sized.  They wouldn't even exchange them and I will never buy them again.  Which makes me suspect the blanks that CMMG uses...at least for the 9mm.

When I thread barrels on my lathe, I use gauge pins to confirm concentricity before threading and depending on the blanks it varies quite a bit but on average most barrels the largest pin I can put in the bore is .347".  I don't think I ever threaded a factory 9mm CMMG RDB barrel so maybe I need to gauge pin the two barrels I have.  1 is new never fired and the other I have up for sale but again that one shoots fine.  So would really need to gauge pin a 'bad' barrel if this is even relevant.  

My 9 is blowback, Endomags.  Built it up before RDB was a thing.  Wanted a 45 for a while but knew I didn't want blowback so waited and came upon a hella deal on an RDB bolt/barrel.  I have modified an insert to a P-Mag and hopefully it will feed and function.  If so, there might be a new magazine option for 40 and 45 and maybe 10mm

I just shot barrel number 3 pictured below this AM and roughly sighted it in at about 35-40 yards and it was hitting everything I was aiming at.  Didn't have time to put it on paper

I need a lathe 😃

God... Don't we all!