Short answers, pulled from the story.
The first recorded use of the term shotgun appeared in 1776 in Kentucky. This designation marked the weapon as a distinct entity within the frontier language of the West before it was known as a fowling piece.
Daniel Myron LeFever introduced the first American hammerless shotgun in 1878 while working for Barber & LeFever in Syracuse, New York. He later patented the first truly automatic hammerless shotgun in 1883 which cocked itself when the breech closed.
Germany filed an official diplomatic protest alleging that the use of shotguns violated the laws of warfare during World War I. The judge advocate general rejected the protest because the Germans had protested the use of lead shot which was illegal but military shot was plated.
The gauge number of a shotgun is determined by the weight in fractions of a pound of a solid sphere of lead with a diameter equal to the inside diameter of the barrel. A 10-gauge shotgun nominally has an inside diameter equal to that of a sphere made from one-tenth of a pound of lead.
The 12-gauge shotgun has a diameter of 18.53 mm and is common for hunting geese and military applications. The 20-gauge shotgun has a diameter of 15.63 mm and is often used for gamebirds such as doves and smaller ducks.
John Moses Browning introduced the Browning Auto-5 in 1900 which became America's first semi-automatic shotgun. This model remained in production until 1998 and followed the Model 1893 Pump Action Shotgun.