Bill,
Spreader tubes for all gauges have bigger "mouth" diameters vs bore diameter. The muzzle end of the 12 ga tube is large even in relative terms. But its still alot smaller than the diameter of the 12 ga expansion chamber. The difference - what the shot column sees - is what matters.
Your perception that the 12 ga spreader produces less nominal choke vs small gauges is right on, and instructive. The NSSA legal 12 ga payload yields adequate 21 yd pellet density using very little choke. So while the .410 spreader approximates mod, it works down from there to about like Winchester's idea of IC when you get to the 12. I can't distinguish Win 12 ga IC from their WS1......maybe at the acquisition price level. Briley's definitions of choke are pretty generally accepted nowadays. Their 12 ga IC is .010 - a bit more than WW's - and it shows.
Clearly, Colonel Cutts understood optimal skeet patterns, by gauge. The term "spreader" here is misleading and has nothing in common with the same term applied to spreader loads. If anything Cutts spreaders tighten things up compared with other period skeet choke designs.
Sam
