I used the metal washer idea with my Encore .250 Sav rifle, 30-06 pistol, 14" Contender .35Rem, and 10" 30-30. I was getting the shots grouping semi good on some and then when they warmed up they were tack drivers. With the metal washers superglued to the forendes all the barrels tightened up on the first 4-5 shots. The difference was remarkable. On two forends I actually used two washers per hole to get more barrel to forend clearance. Easiest improvement to the pistol and rifles since I put new trigger springs in them. I have no fear of the 100yd targets anymore and the two hundred yard targets are suffering dire consequences.
I had used electrical tape, but it started melting to the point it did not work.