The dirty cases are typical of suppressed AR15s. It comes from back pressure from the suppressor. Not a lot you can do really.
Try it without the suppressor and you'll see the difference.
As to "It seems to be inconsistent", without giving specifics of the group size, type of rest and scope and how good the shooter is, there's not much to go on.
Inconsistent as in 1 inch to 10 inch groups? 6 inches to 9 inches? 2 inches to 4 inches?
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