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

I have seen a product that says it can measure pressure inside a gun barrel 
using a strain gage on the outside of the barrel. I have no interest in guns 
but want to know if this is possible to measure pressure with a strain gage 
installed outside the pressured area and get an accurate (or near accurate) 
measurement. What does the strain gage sense in such an instance and does that 
relate directly to internal pressure. My interest is strickly a "want to know" 
question from a dumb person trying to learn.

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