Does drilling a hole in electrical bus to install a lug reduce the ampacity rating of it?

It would take an electrical engineer to give you a more direct answer but here is mine.

When you drill the hole, of course that lowers the ampacity. But then you fill it with a bolt and the lug and nut are touching the bus. And the lug is usually much thicker than the bus to begin with. So in the field we don't worry about it.