Here you go. Early Md. 12's were chambered for 2 9/16" shells.
The Model 12 (introduced in 1912) was the next step from the
Winchester Model 1897 hammer-fired shotgun, which in turn had evolved from the earlier Winchester Model 1893 shotgun. The Model 12 was designed by Winchester employee Thomas Crossly Johnson, and was based in large part on a design by
John Browning. It was an entirely new hammerless design and was initially available in 20 gauge only (12 and 16 gauge guns were not sold until 1914). The Model 12 was the first truly successful modern hammerless pump-action shotgun ever produced. Its tubular magazine was loaded, and the action was fed, through the bottom of the gun. Empty shotgun shells ejected to the right. Depending on the particular wooden transformer plug installed in the magazine, 2, 3, or 4 shells could be stored in the tubular magazine. With its forged and polished steel parts, the ultimate reason for the end of its regular production in 1963 was that it had become too expensive to produce at a competitive price. The primary competition at this time came from the much less expensive
Remington Model 870, which had been introduced in 1950. In addition to price competition issues, the Winchester Model 12 rarely had parts break or wear out, often serving multiple subsequent owners, which further served to limit repeat sales. The majority of "modern" Model 12 shotguns manufactured after the 1930's were chambered for 2¾-inch shotgun shells, only.
Some early Model 12's, however, were chambered for 2 9/16-inch shotgun shells. To add further confusion, some of these early Model 12's have subsequently been modified, with their chambers lengthened to accept 2¾-inch shotgun shells, while others remain in their factory-stock 2 9/16-inch chamber lengths. Careful inspection by a gunsmith is always recommended to determine whether or not it is safe to fire a modern 2¾-inch shotgun shell in older Model 12's.
