The Contact Tip used in a MIG welding torch is a very critical part of the success of the process. This takes the electrical current and transfers it to the moving wire. Various copper alloys are used with alloying elements that reduce the annealing effect of the contact tip getting hot. The bore is a fairly close fit on the wire but it can?t be too close or you have jamming problems, in fact when using aluminium wire, the contact tip bore needs to be larger to help prevent the wire jamming and ?bird-nesting? further back down the system.