A tool that is generally used to slightly elongate a round hole in scrap iron is what?

The tool that is generally used to slightly elongate a round hole in scrap iron is a rat-tail file. This tool is characterized by its long, tapered shape and can be used to refine or adjust the dimensions of a hole by manually filing the material. The rat-tail file is particularly effective in achieving small incremental changes in the size and shape of holes, making it suitable for tasks requiring precision in metalwork.

Other tools mentioned do not serve the same purpose: a drill is typically used to create new holes rather than modify existing ones, a reamer is designed for enlarging holes to precise dimensions and improving the hole's finish, and a punch is used to create holes in a material rather than modifying them. Thus, the rat-tail file is uniquely suited for the task of elongating existing holes in scrap iron.