A common shot pellet size in the U.S.; the diameter of BB shot is used informally sometimes as a distance unit. BB shot has a diameter of 0.180 inch (4.572 mm). See the table of shot pellet sizes
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}