TasoThose appear to be good tools,the bottom one is called a tubing or pin mike, used to measure the thickness's of pipe/walled material,can also be used to measure the outside dimensions of objects that are small enough to fit the throat etc.A good 1" standard mike is very helpful also.If I can ask a question,Are you trying to cut an "O" groove in the face of an action or bbl?Tia,Don