I have done a comprehensive search online and my results have led me to postings on this site made by a user with the handle "SBR7_11."
I recently built a 9mm suppressed AR pistol.
The pistol has a 5.5 inch barrel, 15.5 oz bolt, 12 oz buffer, and traditional carbine spring. It uses a side charging upper and a hahn dedicated mag block for Colt metalform mags. It's a beauty.
The pistol functions perfectly with my suppressor. It is very quiet. The increased recoiling mass is working out precisely as I planned.
However, there is only one drawback. There is a loud noise- albeit only one. The noise of the bolt closing; hitting the barrel chamber face.
Without hearing protection, using a cheek weld, and closing the bolt on an empty chamber, my right ear is hurt by the slamming noise.
If I load one round in the magazine, chamber the round, and fire, the shot is very quiet, the bolt locks back due to the empty magazine, and all is well.
If the magazine is not empty, and the bolt chambers a new round, that chambering is loud. Steel-on-steel slap loud. Not good for my right ear.
SBR7_11 posted that he has cut the chamber face of his 9mm barrel(s) down so that when the bolt closes on a newly chambered round, the bolt is SLIGHTLY offset from the barrel chamber face, by approximately 0.010". I think this requires cutting down the chamber face of the barrel on a lathe and also the barrel flange so the barrel sits back into the upper by the same amount.
This would result in much quieter chambering of rounds, because the bolt never really hits the full chamber face; just the rear of the 9mm case.
Unsupported case length concerns may be justified here, but maybe not, with only 0.010" of gap.
According to SBR7_11, you need 0.135" protrusion of the case measured from the barrel face to the rear of the case.
I am posting this because this is the last piece of a truly quiet suppressed 9mm AR puzzle.
Let's freaking solve the puzzle, together!