I made a cold chisel out of a four inch long section of a 1/4" square file. I use it to create my witness marks when I have installed my breech, and after I have sighted the rifle in on the front and rear sights. By placing the chisel 1/16" back from the edges of the square hole, and giving it a crack with a hammer, you can move metal inwardly to tighten the hole. On the face, the hammer screw should cover the marks.