At the chamber, what is the minimum amount the shell should stick out of the barrel?

I have a 300 BO barrel with a bad chamber and the seller (not here) is not responding.
It is a cheap barrel, but very accurate, so I was thinking about hand reaming the chamber.

But I am not sure if it will clean up without setting it back.

I dont have a lathe and dont want to wrap up too much $ in a cheap barrel.

Thanks