The Model 12 was initially offered in 20 gauge only, which made it the first Winchester shotgun of that gauge and the first hammerless slide-action repeating shotgun to bear the Winchester logo.

Winchester Model 12 History . Over 2 million Winchester Model 12s were sold from the gun’s debut in 1912 until it was discontinued in 1976. The first Model 12s were 20 gauges, followed by 12 and 16 ...

When it was introduced, the Model 12 was only available in 20 gauge, 12- and 16-gauge Model 12s became available in late 1913. A 28-gauge Model 12 was brought out in 1934.

The Model 12 was offered in multiple gauges. In fact, the first Model 12s to hit the market were 20-gauge guns although 12- and 16-gauge offerings came out in 1913 and the 28-gauge debuted in 1934.