Mark, You need a drill press, and a drill vise....a small vice that bolts to your drill plate.
This ain't Rocket Science......far from it!
But you do need to do this as neat and "professional" as possible, because you want to be able to take it back out, and prevent marring and "chatter" if possible....a good clean cut/drill will prevent build-up.
You can do this!
Now why we do it all the time and the manufacturers don't, is anyone's best guess. I do it because it works better.
However, if you noticed in post awhile back, I said I had a Muzzleloader that has no vent liner, I have had this gun for close to 40 years and ignition has gotten better over time...why do you think this is?
I think it's because the flash hole has eroded over time and become larger..on both sides of the barrel.
Uncle Russ...