Seems like the general consensus is that the direct blowback from a 9mm BCG is more violent and will in a sense wear down a standard AR15 trigger.  Seems like certain robust AR15 triggers will work on the AR9 platform depending on if the 9mm BCG is ramped or not.  

I have a lower which I plan to use AR9 most of the time, with the occasional switch with a AR15 upper to use .223/.556, besides the generic "go buy another dedicated lower" option.

Are there any downsides/pitfalls of using a 9mm/PCC trigger lower with a .223/.556 upper

couple of questions you might want to address....do you plan on changing out the buffer for a heavy 9mm buffer or run something like an H3?...., adjust the AR9 bolt travel to just clear the bolt hold open (to help prevent hold open breakage) or not use one?... will the 5.56 footprint 9mm magazines have the same magazine catch height?

I have successfully run an AR9 upper on my registered SBR with a Macon Armory insert and Metalform Colt format magazines with RRA NM trigger and H3 buffer.... I did, however, break off a bolt hold open catch even with a limited travel bolt..that was the end of the SBR 9mm and a dedicated 16" with NFA Glock lower was shot in competition....the SBR was fitted with a RRA NM hammer, which broke with a couple of years of .22lr usage (25,000 or so)  it was replaced with a LaRue MBT 2s.... regards

Yeah I should've put more details into the thread.  I plan to use a CMMG RDS 9mm BCG, which is the same size as a standard .223/.556 bcg.  CMMG states with their RDS setup a carbine length buffer tube and standard carbine weight buffer is recommended.  This would allow me to have a "universal" lower configuration, since it would allow me to not change the buffer or buffer spring at all, when switching from a AR9 upper to a AR15 upper, since the standard carbine buffer and standard carbine spring is in place.

It is a standard AR lower, Aero Precision M4E1.  I will be using 9mm endomags, which is basically a standard magpul PMAG (with endomag internals), this would also allow me a "universal" configuration.

my AR9 for USPSA PCC was changed to a CMMG radial delayed upper, but with a New Frontier Glock format lower... the bolt is similar to a 5.56, but the pointed engagement lugs are very sharp, and mine caused a problem with getting the correct installed height of the magazine... upon cycling, the sharp lugs would drag on the next to present cartridge or the magazine feed lips... others were able to solve the problem by simply changing the magazine catch height, I was not... additionally at about 1200 rounds, the polymer helper plug in the extractor spring hardened causing feeding problems... a member on the Brian Enos board, Memphis Mechanic, suggested using a BCM heavy duty spring and deleting any helper plug or Viton "O" ring which I did...Amphibian, on this board has found the answer to poor ejector springs.... regards Les