How can I determine the gauge of my L.C. Smith shotgun?

The Hunter Arms Company did not mark the gauge (bore size) on the receiver or barrels. Open the gun and measure across the chamber. Do not include the recess for the rim of the shotshell.

    12 gauge is 0.810" or about 20.6 mm
    16 gauge is 0.745" or about 19.0 mm
    20 gauge is 0.698" or about 17.7 mm

Gauge size was originally determined by the number of equal sized lead balls that it took to make one pound of lead. It takes 12 lead balls 0.729 inches in diameter, 16 lead balls 0.662 inches in diameter, or 20 lead balls 0.615 inches in diameter to weigh a pound.