It is for free floating the barrel. I have found that rubber washers work best. I get them at home depot for less than 10 cents a piece and they make a huge difference. All of my barrels will shoot noticably better with the barrel free floated by the washers.
Its easiest to assemble if they are just big enough to hold the fore end screws in place while you are holding the foreend upright. I use o rings and a metal washer but the theory is the same. Eliminate contact between the barrel and fore end.
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