Ian,
Thanks for the question, I really enjoy answering these, keep them coming, please. This is in the realm of what is known as "primary research" in that these is nothing published that would allow us to go to the library to read about the details of making 18th c guns. So probably not more than a handfull of folks know how it was actually done. Perhaps a better question is "what is the resulting hole in metal like when formed using only 18th c tooling and processes? " Of course, this includes the flash hole. Just as an aside, this is one of the most difficult holes to drill as the length/diameter ratio makes it a very deep hole; it is tiny so the drill and reamer bits are very easily broken; and it is often drilled at a slight angle so both ends are positioned correctly.
Here is a photo of an original bow drill bit, the kind used to drill small holes, such as the flash hole. The fish tail shape of the bit is to allow for wriggle/wobble of the tool during drilling without breakage. This type of bit will not result in a well controlled hole either in diameter or in straightness, as we are used to getting with the modern twist drills.
The process of forming any hole in metal with accuracy involved first drilling the hole with this tool, followed by reaming to size with the tapered reamer. As far as I know, small diameter holes in metal in the 18th c were always slightly tapered. Tooling did not exist to give a smooth, straight, constant diameter small hole in this period. Therefore, the flash holes in the 18th c, and all such holes were tapered, the tooling and process did not exist to make them otherwise.
Larger diameter holes, such as the barrel bore were honed smooth, straight, and of constant diameter using an adjustable armory reamer. The armory reamer has straight cutting edges on the long sides of the tool which allows the barrel bore to be formed without the taper of the smaller holes. Here is a photo of the armory reamer.
I have never found an armory reamer that was anywhere near tiny enough to form a flash hole. As a challenge to those who wish to make really accurate reproductions of the 18th c guns, the flash hole must be very slightly tapered!
Jim