So I seem to have run into a problem but it is not really a problem at all because this is why Faxon produces accurate bbls (I think).
I got one of the newer 9mm bbls with the newer muzzle thread pitch...I ordered right when the change occurred, and had expected to get 1/2-36 but that's not a big problem, so I can deal with that.
But my rounds weren't really sitting flush-enough in the bbl chamber, and I said oh well and shot anyway. It was fine, and any overpressures were handled by the mountain of metal around the chamber. But I did find a lot of noise coming out of the breech, i.e., not hearing safe noise (with a suppressor). I could be wrong but I think the bolt face wasn't "closing" or fully mating with the blowback bbl. Even dropping rounds in the bbl before I put it on the receiver did show the case body was sticking out a tad. ...this was using 124 FN bullets seated at 1.060" and 1.080"; it was just the supply I had on hand from prior daylong reloading sessions.
I brought it home and did some testing to see what my max COAL would be for two different 9mm bullets I shoot....should have done this beforehand, of course. Duh.
For the x-treme 124 FN (which is a short bullet at 0.525" bullet oal), I got 1.040 before I hit the lands. If I back of the land by 10 thou, then I'm at a 1.030" COAL with the FN.
For the x-treme 165 RN (which has a 0.735" bullet oal), I got 1.150" when touching the lands. If back that off, then I'm at 1.140" COAL with the RN. I would imagine the that other x-treme RN bullets would likely produce a similar max COAL since their nose geometry is largely the same. Lighter weight RNs, of the same nose geometry, would give you more case capacity.
A 1.140" COAL is not at all "crazy short" and there are benefits to this (like uhh accuracy). So I am not complaining or finding fault, and I certainly could have messed up my exact measurements, but I suppose my point is that the cut of the throat/lands brings the lands pretty close to the bolt face. On balance, this is probably a very good thing, because....in a rifle caliber where I am limited by magazine length, like the Grendel or Creedmoor, I would LOVE this problem...looooove it. Esp since its nitrided and unlikely to wear away at the throat quickly...i.e., have long accuracy life where my mag isn't preventing me from getting ultra close to the lands. This is the sole reason I hate semi auto guns chambered in accuracy cartridges, and Faxon seems to have gone ahead and completely fixed that for me. I definitely don't hate you guys, but doing this in a 9mm is killing when you're not doing it in Grendel or Creedmoor.
In contrast to the typical situation with Grendel/Creedmoor ARs, in this instance, my PSA metalform mags allow me a huge COAL (>1.200") but my bbl limits me in COAL. The only possible problem I see is possibly a situation where I cannot seat a bullet at a long enough COAL such that feeding problems arise. I am not sure that this will occur at all, especially if you're using RNs and seating to 1.140 COAL, but idk. It would be worth checking out. If I have feeding problems with my RNs at 1.140" or 1.150" then I will make a point to followup here.
Sidenote...I completely eliminated my feeding problems with FN bullets by seating the rounds in the magazine a little far forward, in the magazine. If the FNs were seated back against the mag body, then I would have failures to feed. And the rounds stayed seated forward like that (in the mag) even after firing several rounds.....and I should note for readers that this bbl has nothing directly to do with any feeding problems that would ever arise because feeding is a magazine and cartridge and feedramp problem, never a bbl problem...unless the bbl chamber is preventing you from have a sufficiently long COAL, which is doubtful at 1.140" COAL. Oh and these 9mm AR bbls should not and cannot have feed ramps built into them, of course.
But I think this is really what accounts for the accuracy of these bbls, and yes it was accurate. Surprisingly so. If Faxon does start making Grendels and Creedmoors, please remember this or take it into account in whatever way your expertise determines is best.