I have been torn between a Sig MPX and building an AR9.  Right now the AR9 concept seems more practical.  Sigs are $1800 and will still need expensive work for top performance.

This is for 3 gun by the way.

Now AR9, I would like to just buy a JP 9mm, but that's also $1800 and I would still want to change trigger and handguard.

Next step down for me is to buy the JP bolt, JP SCS, and JP barrel, but put all that in my own cheaper receivers and add my own trigger and furniture.
Does JP sell their 9mm upper and lower receivers stripped?  I could not find them.

Then I guess I would have to wait on the Quarter Circle receivers to come back in stock.

Since QC is so hard to find, can I use any 9mm upper/lower combo and get reliability?  I know a lot of people who built glock PCC's on PSA 9 lowers have had trouble, though I don't know of what nature.

Trigger will be Hiperfire 24C btw.

my PCC is a New Frontier Armory (NFA) Glock lower and uses a NFA bolt with the AR15 extractor and comes with a bumper...I don't have a need for a LRBHO,...additionally I have a PSA (NFA) lower that other than cosmetically is the same.. a couple of passes with a file to bevel the edge of the ejector where the bolt runs close... not a problem with the NFA lower for other than operator error (not shouldering tightly) for approximately 3000 rounds.... I bought the NFA complete lower for less than the price of a stripped QC10 lower....YMMV

You can buy the JP Lower receiver, I bought a CMT upper which paired very well with the JP lower. Though any Mil spec upper should work just fine. The JP barrel or Taccom is super, Faxon is a great option as well.

Here is my build list of 2 builds, a budget and high end like you want to build. http://recoiljunkie.com/pcc-budget-and-build-sheet/

If I was building another, which I am actually. I'd lean towards the taccom barrel, bolt and recoil system. The receivers like any AR is just about the options and how you like the controls.

I used a QC10 GSF lower and a CMT upper. I wanted a QC10 upper but got tired of waiting. With the JP SCS and bolt with a Macon Armory barrel and rail it runs great. After a short break in the LRBHO is 100%.

FYI. Our club is running 3 Gun separate from PCC matches (Greenville). Simplifies setup somewhat. So is the adjacent club (Belton).

I use a 9mm MPX (115 gr at 1265 fps) and wore our a Hyperfire 24C in 3500 rounds. Hyperfire sent a new one at no charge. They said Taran Butler had 8000 rounds thru his without a problem. I guess I will find out! Maybe a bad heat treat job??

Building mine with a PSA complete glock mag lower (on sale a month ago for $189 delivered), just got an upper (inexpensive Anderson lightweight no FA from aimsurplus for $35), 16" NFA barrel ($99, also from aim).  

Still need bolt, going to get a Spinta blem for $115ish shipped, muzzle device ($15) and handguards/barrel nut.  Thinking about a $40 free float tube from fleabay that comes with barrel nut.  Planning on putting a TRS-25 on top (I have 2 already), either with an after market riser or with the built-in-to-sight AR riser.  Should be complete for just over $500.

If I didn't work in education - or if I worked in a different department - and had the extra $ the side-charging lrbho-for-glock-mags upper from Joe Bobs (Spartan brand) is really tempting....

if you find out what trouble they've been having it would be interesting to post... I've had no issues with my homebuilt from no name build pistol kit parts and the PSA 12 in barrel with mlok rail and kak flash can.  It's run great, so far with between 300 and 350 bulk 115 gr. rounds of all different makes , but then I also do not use it at competitions.  I wouldn't be scared away by the glock PSA lower so far it seems pretty top notch. You can get the bolt hold open in the upper from NFA that is supposed to work with glock mags.  I'm using the smg 33 round mags and so far so good.  

Please keep us up to date on the build.

I don't know what the problems were, just a few people had chokes at local 3 gun matches.  Not sure all were PSA either, but suspect reloaded ammo issues.