Copper, and aluminum are excellent conductors of heat. If copper were to be wrapped around the blade, much like a very thich habaki, the heat would not transfer past that point, therefore protecting the temper of the blade. A typical "heat affected zone" from welding extends 1- 1 1/2" from the toe (edge) of the weld. the heat affected zone is the area in which the mechanical properties of the material have been heated to the point where the hardness, ductility, and tensile strength have been altered due to the application of heat. With the right welding process, and heat affected zone control, the affected area could be reduced to as little as 1/2"-1" (12mm-25mm). The heat affected zone is typically characterized by a dark changing of colors, sometimes resembling a rainbow of dark golds, dark reds, and blue/blacks. With this small of an area needing repaired, I see no worries with affecting the surrounding areas. The main question is, should a repair of any type be made. The bullet hole is a part of the history, but that part is better determined by someone with greater knowledge than myself.