Yes, I have the original saker 556 with the 5.56 flash hiding endcap. I have never shot it under NODs so I can't comment on that, but it does reduce flash in my 11.5 gun especially when using bulk ammo like the american eagle 5.56. When I was shooting Gold Dots, MK262 magtech, and federal fusion I honestly didn't notice much of a flash difference (because there is little to begin with the higher quality rounds). Note that I was using the full sized 5.56 can. I would expect to see a bigger difference when using a k can.
For example, I have a sandman k on a 13.7" gun and the flash with the AF 55 grain is significant. I'm looking forward to getting the flash hiding cap for the k can.