In his definitive book "Winchester's Finest - The Model 21" author Ned Schwing states on page 96 that all 21's in 12-16-20 gauge were chambered 2-3/4" from the very start of production, except for 28-gauge initially 2-7/8" and then 2-3/4" after the War. That excludes of course those factory chambered and stamped 3-inch.
Some additional comments: often modern "gauges" that depend on seating in the taper of the chamber and correlating to its length can give inaccurate readings owing to their manufacturing tolerances and/or chambers cut with reamers that were worn or with an inaccurately ground taper. Also, gauges that bump into the start of the forcing cone can give inaccurate length measurement unless sized exactly to period SAAMI front chamber diameter and even then you've got a +/- tolerance on diameter. A few thou in diameter makes a big difference when trying to measure length. Then you have the budget techniques using cut-off shells, dowels, machinist scales and rolled up postcards. Best ime for those that fret chamber length, shell overlap into the cone, etc is a gauge that's accurately made on the lathe between centers and sized dead nuts to absolute minimum of that SAAMI front chamber diameter. With such gauges I've measured every 12 and 16 Model 21 I've ever owned including Prewars and found all to be on size or a tad longer than 2-3/4" or 3" if stamped for 3-inch shells.
Top 10 and 12, Bottom 20 and 16
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