Eodinert: I'm just trying to get subsonics to run period! I checked the port and it measured just at 0.120" so being undergassed doesn't seem to be the issue. I also ordered a reduced power buffer spring, so hopefully with the H2 buffer that will solve the problem.
Otherwise, I'll just have to find a 16" barrel with a pistol length gas system. However, given that everything is currently out of stock with no backorders in sight it may be quite a while before I find one.