Reducing blowback with a suppressed AR?

[SDguy]

My gunsmithing skills are minimal. I have two of these that have been very beneficial to overgassed guns. As noted above, they are not as refined as an adjustable gas block, but they can certainly improve one's situation quite readily.

I would buy again.

Is the adjustable glass carrier meant to be used with an adjustable gas block or instead of? Pros & cons?