I have early made Odin 18" 6mm ARC barrel, but no suppressor, as I live in a shit hole state. I had to open the gas port to #36 from size #38, doing that has really let it run any ammo w/o issue, and its tamed by an adjustable gas block. The cases were no more sooty than other ARs I run.
Went thru the same mental gymnastics the OP went thru on headspace. The factory Hornady ELD-M and Black BTHP were stretching 11 to 14 thou. after first firing. As someone else mentioned earlier, if the factory cases are toward MINIMUM SAAMI specs, and a CHAMBER is on the longer side of SAAMI specs, it can really stretch the cases out from their initial factory size.
My PTG CHAMBER Go gauge in Wilson CASE gauge is right at min. CHAMBER spec, the NO-Go gauge is +.005 over the Go. Most factory Hornady ammo is -.003 to -.005 below SAAMI max CASE spec. My bolt/odin closes on both PTG Go and No-Go gauges, and adding single layer of Scotch tape to No-Go, will close on it with a really hard thumb press, but a field gauge will not. I'm guessing the chamber is 7 to 8 thou longer than SAAMI min chamber.
PTG actually has some good info, about factory AR chamber headspace, that can be found here.
https://pacifictoolandgauge.com/content/19-ptg-headspace-gauge-info
"I have a lot of customers calling me about their hand guns or gas guns closing on the No-Go, such as an AR-15 closing on a standard No-Go gauge.
Remember most gas gun manufacturers run their headspace a little longer for rapid firing and to have the ability to perform with a dirty chamber.
If a precision gunsmith chambers a bolt action barrel or an AR for you, he will most likely ream until the bolt just closes on the Go, but a +.005 No-Go should work as a No-Go.
Remember, most manufacturers chamber their AR-15’s and other gas guns to +.002 for Go from SAAMI Min to +.007 from SAAMI Min for their production No-Go. I always check my gas guns with a military 11 piece incremental set of gauges to find out the true depths of the headspace. "