You're probably way outside the normal operating window running an 18" barrel rifle gas system, with a bullet moving 4,000fps. It's not dwell distance, it's dwell time, and you don't have any at 4,000fps.
That gas port is going to be very large, with your little 20 cal bullet running over a significant size pot hole in the barrel.
Unfortunately, the ARFCOM group-think is so thick, Wilson would never be able to sell one with a correct mid gas system.
Tony