I built a 9mm upper around the CMMG RDB bolt.
I bought the bolt/barrel on sale for just over $300.
Mine has the 8 inch barrel.
I bought a larue 7 inch handguard for $70 and already had a milspec upper receiver.
Then I bought a 3 pack of CMMG pmag conversions that allow you to shoot 9mm through normal pmags for around $100.
I used it with a "pistol" lower I already had in inventory.
It's actually one of my favorite guns to date.
As a side bonus, it's so quiet (suppressed) using my 147 grain handloads that my wife really took a liking to it -
Now she's totally trained on the AR15 manual of arms, and comfortable with all my ARs
(It took her a while to warm up to centerfire rifles because of a "bubba" type scope eye experience when she was young.)
It's very comfortable to shoot without hearing protection with my rugged obsidian 9.
The only quieter centerfire I have is my henry model x shooting 38spl through the obsidian.
With a red dot and WML it makes a great nighttime pest exterminator that won't bother the neighbors.
Wouldn't be so bad on bad guys either, but I prefer 77 TMKs for that
Being able to use any milspec lower is a huge plus for the CMMG system with their endomag conversion.
I also shot it back to back with a buddy's PSA 9mm AR - we both agreed my CMMG build had a much softer recoil impulse and was drastically quieter at the ear suppressed.
I'd do every part of the build again exactly the same.