To use witness marks you need two marks on the barrel. One for depth and one to line up to mark on receiver. Will look like a T on the barrel. With the thicker lug you cover the mark for depth thereby changing your head-space. The pointer DenisA has made up would work due to being able to reference on the witness marks away from the barrel nut therefore not covering the mark used for head-space. Remember there are two marks. One to line up on and the other to check head-space. Keeps you from going a turn short or a turn long.
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