I think it will be excellent for an 18" 6mm ARC. He actually designed the full-size Polo around both the 5.56 and the 6mm ARC (this is what I was told by
@AndrewKing who is the owner of Otter Creek). He wanted the 556 can to have enough volume to properly suppress the larger 6mm ARC case, and still sound good. I have not shot mine on my 6 ARC bolt-action yet, but I might do that this weekend to see how it sounds, and to get some more video footage. And compare the full-size Polo to the Hydrogen-S on it.
I don't have adjustable gas on my M4 and it's gassed perfectly. Brass ejects at 3 o'clock, and it's not gassy at all. Here's a video showing it being ran on my rifle... Notice the brass slinging perfectly at 90º and hitting the wall.