Re: can pressure be measured out side the contained pressure area?

Pressure, which is the force exerted on a given area, is measured by 
changes effected on the material constituting that area, e.g. the interior 
surface of the gun barrel.
From what you have described, I would say the subject strain gage senses 
the elastic expansion of the gun barrel diameter as it reacts to the 
changing pressures within it. The strain gage measures the initial 
diameter of the barrel, and then upon firing the gun, the gage measures 
the maxima of the diameter of the barrel as an indicative function of the 
maximum pressure acheived within the barrel.
The actual numeric result of said pressure would have to be a function of 
the thickness of the barrel, the elasticity of the barrel metal, as well 
as its initial diameter, among other factors.