So I've owned two DD Mk18 uppers now.

1) Post 2017 RIS II.  Gassed strongly, but not unmanageably excessive.  Carbine tube/milspec spring/H3 has it running well; 4-4:30 ejection without can, about 2-2:30 with RC2.

2) Brand new production, just received RIS III upper:  Seems brutally overgassed.   Carbine tube/milspec spring/H3 has it ejecting forward without the can and further forward with it.  An A5 lower with Sprinco green/H4 buffer has it ejecting about 3:00 with no can, and 2:00 with RC2.  Also gas to the face seems pretty significant, more than lighter gassed uppers.

I don't have any experience with pre-2017 uppers which were said to have larger gas ports than post-2017.  The RIS II is sold, and I don't really want to un-pin the gas block on the RIS III just to try to guesstimate the port size, so I'm wondering if anyone has experienced the same, or knows if DD went back to the larger gas port spec?

Either way, I'm thinking this may turn into my first rodeo with BRT gas tubes.  I don't want to have to run it with an A5/bordering on PCC super heavy buffers to have it run well.

~ '17 is when the GP diameter was reduced, as you stated. From .081" (IIRC) to .070" (fairly certain).

I'd post HERE for info. Marler5811 should give you the lowdown.

Ah, I guess that would make sense.

BRT tube ordered, specified milspec carbine spring/H1 buffer/unsuppressed.
I will eyeball compare tube port sizes at least.

Assuming it runs well in the above configuration, I should have some room to add buffer weight with the can if needed.