I thought so too, having always heard the 12 bores were chambered 2 5/8 for 2 3/4 shells. When I looked at TPS, however, the factory chart from 1920 clearly shows 2 1/2" chambering as standard for the 12 ga. See Vol. 2, p. 519, Fig. 12-18. I measured them with both a fixed diameter Walker gauge, and my inside dial caliper. Definitely 2 1/2 inches.
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