I drilled out the gas port on my 16" AR10 & put a switchable gas block on it. The gas length is considered Armalite's AR10 Carbine mid length, which is about 0.20" longer than a normal AR15 mid-length gas system, so it is pretty long for subsonics.
I emptied the buffer & installed a Wolff reduced power buffer spring & it would almost cycle, but the BCG would stop over the mag, just before closing.
I had the older-style bolt carrier with the flat bottom. Their newest one is beveled/sharpened it so it will slide over the cartridges in the mag with less resistance. I ordered the newest generation & it cycled. I think I may have trimmed the buffer spring until it would cycle.
I ordered the 4-5 different subsonic .308 loads that Midway sells & they all worked fine. The extreme shock was Much louder than everything else for some reason.
I never shot it for groups or handloaded anything for it ..... too many other projects I guess. The 7.62x39, or .300 Whisper are much more practical & don't have as big of a problem with having too much case capacity.