I have a M1S upper with carbine gas. As mentioned above, I have to use AA-1680 to get the subsonics to cycle. But once I switched to 1680 I was able to work up a load for subsonics that uses the relatively cheap Hornady 150gr FMJ, in addition to my loads with 220gr bullets. It also cycles supersonic just fine.
I decided to find a lod for my set up, rather than add an adjustable block. But if I was ordering just a barrel, not an upper, I would probably have gone with pistol gas and an adjustable gas block.
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