That Thumb to middle finger will "VARY" quite a bit dependant on the size of one's hand. I have rather small hands & just took a look at the circle made when I touched my middle finger with my thumb. That would be a small gun indeed.

I have known a few people with hands so large & long fingers who could likely do that on a 12 gauge. An actual circumference in Inches or MMs would be far more meaningful.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra