There would be no reason for chamber pressure to be any differrent in a chamber reducer than in a chambered barrel. I would very seriously doubt that the barrel chamber walls are REQUIRED to absorb any stress from the chamber pressure; the reducer should be capable of being fired without damage outside a barrel. Depending on the tightness of the fit between the reducer and the chamber, the barrel might strain as part of the reducer-barrel system.
When the wad transitions from the reducer to the barrel bore, there should be a pressure drop. So, chamber pressure should be normal and barrel pressures lower.
There is no reason that strain gauges can not be glued all over a barrel; gauges are cheap. However, measuring/recording equipment is not cheap. Operating mulitple channels is expensive. To get good data with a single channel set-up would require the repeated firing of statistically significant numbers of shells for the various locations.
If one really wanted to measure the chamber pressure of the reducer-barrel system with a gauge on the outside of the barrel chamber area, one would need to run a series of known calibration pressures. One way this could be accomplished would be to seal the barrel in the front area of the insert and subject the system to static hydraulic pressure measured by a known accurate pressure gauge. Strain gauges further down the barrel could be calibrated by this method, taking care to seal the barrel such that no part is subjected to above design pressure. Such a system would allow measured pressure-displacement curves.
All it takes is time and money!! The know-how is there.
