Larry
I hope I can make it clear enough.
First, it may have none. I say that because it helps most but not all. Some rifles it doesn't matter. With that said, I did mine before firing a shot through it.
When I did mine, I used a piece of 1/8" thick magnetized rubber strip. Why, cause that was what I had, partly. Secondly, I felt, unlike an "O" ring, it would not crush easily and be less subject to overtightening issues.
The magnetized rubber I had is about 1" wide. I cut a piece just long enough to fit in the recessed area of the forearm. I then used a hole punch to trim a hole large enough to allow the barrel lug to go through it. I made sure the piece lay flat in the recessed area and replaced the forarm and screw. I tightened the screw snug enough that I felt it would not back out, and that was it.