Twenty gauge nominal bore diameter is .615"
Admiral Thomas Perry's 20 gauge Damascus DH, a 1908 gun, bores measure .619" and .620" which seem just a whisker oversize, but who knows?
I can very well imagine Parker Bros. may have polished or honed these bores... being Damascus they may have had minor imperfections after the first boring that may have needed a clean-up.
In my experience in the field of Volvos a four-cylinder block would have been fitted with three "C" sized pistons and one "D" due to the fact that the fourth hole was bored with a cutter that had already cut three holes and was dulled to the point that the fourth hole (cylinder) needed extra honing, hence the one larger piston.
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"I'm a Setter man.
Not because I think they're better than the other breeds,
but because I'm a romantic - stuck on tradition - and to me, a Setter just "belongs" in the grouse picture."
George King, "That's Ruff", 2010 - a timeless classic.
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