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No doubt that I'm off with the case head measurement. I just roughly measured a 12g with crappy plastic ruler and rounded it to the nearest mm. The working should be solid though.

If you're...

12g case head roughly 1.9cm diameter.
1 inch = 2.54cm
1.9/2.54=0.748 inches

Area of a circle
A=pi x r^2
pi x (0.748/2)^2 = 0.439 in^2

12g case head about 0.439 in^2