About 40 years ago Dixie had a bunch of smooth bore barrel about 43 caliber. They were only $10 each so I bought a few. After seeing the Whitworth rifle I thought it would be a good idea to try making one. So trying to figure out the hex bore was fun, seems the flats are 1/2 the bore diameter. Easy enough, just make a cutter box at bore diameter and a cutter at half the bore diameter wide. Now to figure out how to make a bullet mold.
hex bore.pdf