I am looking to start ordering some parts for my build here in a few days but want to make sure I am on the right track.

Cmmg complete MK9 lower with stock.  Gun shop close to my house has it for a decent price (352 with TTL).

After that I got two routes.

RRA A4 complete upper (they now have ramped bolt so should work with my CMMG lower hammer?)

or go the build route and get the gun 100% the way I want it

RRA A4 upper
RRA m4 carbine barrel
CMMG bolt (i figured this would work better with the CMMG lower I have read some about the bolt catch hitting?)
quad rail setup

Any comments on the CMMG lower and bolt catch issues?

Do yourself a huge favor and purchase the JP 9mm Bolt and Silent Capture Spring set.

The JP bolt is ramped, and the Silent Capture Spring runs very smooth.

I had a dedicated CMMG Mk9 lower. I couldnt get it to run.
Get one of these http://ammarms.com/?p=238
You'll be much happier in the long run. They made them for Spikes to re brand them as Spikes Tactical.
Far as "I'm concerned they are the best drop in block out there.

Quarter Circle 10 makes lowers for Glock and Colt style magazines. No blocks to worry about, use regular AR parts mostly. I have the small frame Glock one with a 9MM and 40 S&W uppers I built myself. Works great. The JP Bolt and spring are good advise bolt is ramped and narrowed for Glock magazines.

Im running the CMMG MK9 lower with a X Products 9mm SCU Outlaw upper

No issues with the bolt catch. I really like the CMMG lower. Here's a tip; get your mags from Brownells. They are 1000x better than the cheap CMMG mags or whatever is floating around out there.

But the bolt catches open with the brownells mags, x products drum, and the CMMG mags.

Are you using the JP spring for the 9mm and the.40? What bolt for the .40?

Does anyone know if a 9mm bolt could be opened up for a .40(specifically a JP 9mm bolt)?

QC10 makes .40 bolts that are already ramped and ready to go, IIRC.

My 40 bolt was opened up by ADCO, excellent job and fast service.

Ended up getting a JSE upper...I could not put it together for the price..

YHM upper
16" wilson hbar SS barrel
CMMG bolt

Lower is the a complete CMMG from buds got a good deal as well.

How are the PSA metalform mags?  I am gonna grab a few of them.

http://palmettostatearmory.com/index.php/ar-9/psa-9mm-smg-magazine.html

PSA's 9mm Metalform mags are GTG.  And $25 is a good price.  

Metalform is the gold standard for 9mm AR mags.  The only thing bad about these mags is that ugly PSA logo.  



 

Yes their logo is stamped on the bottom but I can't imagine anyone is staring at the bottom plate on their magazine.