Jerry,
When a chamber is deepened, it is essentially moving the forward corner of the chamber/forcing cone forward. Since the outside barrel contour has a taper, what ever distance the chamber is deepened will result in a reduction in the wallthickness in that area. Dividing in half the difference in diameter between where the original chamber end is and where it will be after deepening will give you the reduction in wallthickness. I suspect the difference in wallthickness is in the .010-.015 range most times that a chamber is deepened less than a 1/4". A few measurements should tell you.
