Admittedly without checking, I think it is possible that a .410 bore chamber is around 36 ga. while the bore diameter, ahead of the chamber is around 67 ga. Seems reasonable to me- what say-you?
I keep wishing I could see Miller type one of his mathematical formulae posts in answer, but since he is no longer with us I will attempt.
The numbers I come up with don't bear that out, Mike. A .410 chamber diameter tapers from .4811" down to ..4630" just before the forcing come begins. Doing the math we get about 47 balls per pound at .4630", and about 41-42 balls per pound at .4811", so that would be saying the chamber bore is somewhere between a 41 and a 47 gauge. Neither is close to 36.
