I might be offering an overly complicated answer (positive that has ever happened before ) as to how this measurement can be obtained but, what about using the measurement tool bar in a PDF?
All that is necessary to do so is to take a picture, much as you have done above and then convert it to a PDF so that the measurement toolbar can be used to obtain a linear value. Of course it would also be necessary to include a measurement scale standard so that the correct correction to scale can be made. The measurement scale standard must also be placed in the same focal plane as the measurement to eliminate/minimize error.
I use this very same technique in order to determine the average expansion diameter of recovered bullets that have been fired in water tests. The process is similar. After conversion of the image to a PDF, I use the PDF measurement toolbar to obtain the area of the test bullet's expansion face and then solve for the diameter accordingly after adjusting for scale. It is a process that is much, much more accurate than getting out a set of dial calipers and measuring individual maximum and minimum expansion values to obtain an average diameter.