I built a 6.5X47 Lapua with an octagon barrel and wanted an octagon banded front sight. I knew that I had to make my own. I had a piece of 4340CM and started whittling metal off of it.
I actually made 2, but the first was done with the tip of the endmill and left too much tool marks. The second was done by the side or flute of the endmill. A friend had a wire EDM and cut the octagon ID.