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Not very.
There are 25.4mm to 1 inch. And when you're dealing with your heads you're interested in the VOLUME of those millimeters which means you're dealing with CENTImeters. So here's one way to get a feel for them. See the tape measure? See the markings? Notice that this particular tape measure give you both inches and millimeters. Very handy!
If you'll click on the image it will fill your screen, flop off the edge of the desk and run all over the floor. Honest! But the larger image will give you a chance to evaluate the
size of a CENTIMETER, which what you'll be working with when you cc your heads.__R. S. Hoover