That's part of the reason to go with factory rifles if I can. They're designed to work well together and they warranty the gun. Barring that duplicating a factory configuration is a safe bet.
http://www.bravocompanyusa.com/BCM-U...kout-s/207.htm
Not many suppressor are reflexive these days (AAC M4-SPR or Ops Inc. models). I would shop suppressors and uppers.
Interestingly, it seems the 12.5" BCM is a pistol length gas as well as the 9".
What barrel length are you considering? Any ideas? Are you very familiar with suppressors?
God bless,
Brandon
ETA: Just saw Silencerco Omega. The mount indicates the suppressor ends pretty close to the beginning of the flash hider. I think others can give specifics on the Omega.
https://www.google.com/search?q=sile...JrVzbGt0aqM%3A
Were you thinking of a 9" or 12.5" BCM?