Some have to make their own drill bits due to special length requirements. Gun drills come to mind not just for making rifle barrels but having to bore a hole through a long solid bar. There are manufactures of these to be sure but sometimes it is hard to justify the sometimes-huge expenditure to purchase a ready made bit but then again, many gun drills will have a single straight flute with a hollow fluid flow cavity for high pressure flooding. These bits sometimes use indexable inserts or brazed carbide for the cutting edge