Asking gun builders this question is like asking 300 guys and 300 ladies what their favorite pickup or car is.
Lots of different ways. Some better than others. I have tried every method know to mortals, and some only known to the gods, and I have established a method that works. Took several years of experimentation, and both purchasing and making special tooling.
The key is the gent doing the chambering must totally understand precision machining, know how to accurately put the barrel in the lathe, must have good equipment, and must know how to use the lathe.
Feeds, speeds, coolant, type of reamer, who made the reamer, type of barrel setup, lathe alignment, are all important.
There is a lot more to using a lathe then just flipping on the power switch.
I would not begin to describe my methods, would take a lot of photos and many, many words.