How old is a Winchester model 1400 MK11 20 ga semi-automatic shotgun serial?

To determine the age of a Winchester Model 1400 MKII 20-gauge semi-automatic shotgun, you need the serial number, as Winchester used a specific dating system. Generally, these shotguns were produced from the late 1960s to the early 1980s. You can reference a Winchester serial number lookup guide or contact a gun historian or collector to get an exact year based on the serial number.