When I'm drilling/tapping a rifle receiver for scope mounts, I might only have a blind hole 5 or 6 threads deep to work with. Getting every bit of that hole threaded requires a starting cut with a plug tap, followed by a bottom tap to chase the last few turns to full depth. On a 6-48 thread, the trick is getting the holes fully threaded without breaking the tap off in a 100-year-old receiver that might cost a couple of thousand dollars!
Jerry
