Can you tell me when my Winchester, Model 12, 20 Gauge, Shotgun, S/N: 326845, was made?

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I am going to make a SWAG and say your Model is a 1400, and the "n" in the serial number indicates it was made after 1968. There are no good date lists for Winchester arms.

The Winchester Model 1897 evolved from the Winchester Model 1893. The Model 1897 and 1893 were both designed by John Browning. The Model 1897 is an external hammer shotgun lacking a trigger disconnector. This means that the user can hold the trigger down while cycling the shotgun and once the action is returned to battery the gun fires. The gun itself is classified as a slide action pump shotgun. It was the first truly successful pump-action shotgun produced. Throughout the time period the Model 1897 was in production, over a million of the type were produced in various grades and barrel lengths. 16-gauge guns had a standard barrel length of 28 inches, while 12-gauge guns were furnished with 30-inch length barrels. Special length barrels could be ordered in lengths as short as 20 inches, and as long as 36 inches. Along with various grades and barrel lengths, the Model 1897 came in two different chamberings. One was the 12 gauge and the other was the 16 gauge. The shells should be of the 2-¾ inch or 2-? inch model. Any shells larger are not recommended. An average Model 1897 held 5 shotgun shells in the magazine tube. After including the one shell that could be held in the chamber, the average Model 1897 held a total of 6 shotgun shells. However, this would vary from grade to grade. When working the action of the Model 1897 the forend (fore grip) is pulled back, forcing the breech bolt to the rear which extracts and then ejects the spent shell while simultaneously cocking the external hammer by pushing it to the rear. When the forend is slid forward again, the breech bolt pushes a fresh shell into the gun's chamber and locks into place. This action of sliding the forend back and forth (pumping) is why the gun is classified as a slide action (or pump) gun.

Your model 12 was made sometime in 1915 probably early first quarter as Winchester site states that 1914 ended with serial no 79765 for the model 12

what model number? try here if its an earlier shotgun:
http://www.winchesterguns.com/support/files/images/wfa/2012-All/2012-Articles/Winchester-Manufacture-Dates-by-Year----2012-Scanned-Documents.pdf

Need to know more info. What gauge is it? What model is it?

1897

Nicknamed, the Perfect Repeater at its introduction, this shotgun se tthe standard for pump action shotguns throughout its fifty one (51) year high rate manufacture and production life. From 1912 until the shotgun was first discontinued by Winchester in 1963, nearly two million Model 12 shotguns were manufactured and produced in various barrel lengths and grades. Initially the M12 was chambered strictly for 20 gauge, however 12 and 16 gauge versions came out in or around 1914, and the 28 gauge version of the Winchester Model 12 came out in 1934. A .410 version was never produced; instead, a stripped down variation of the Model 12, commonly known as the Model 42, was derived directly from scaled drawings of the model 12 and produced instead. The year of manufacture for serial number 787927 is 1939. hope this helps! im pretty sure that this is right. check it out on the web.