This is why I've gone to patterning my shotguns the last several years. As a younger man, I was content to announce a gun's choke as whatever was roll marked on the barrel. Then, I began trying the measurement game and found bore and choke diameters varying pretty wildly from industry standard, as indicated by Brian and Todd in their posts. Eventually, patterning came to be my preferred way of actually knowing what I had. It was eye-opening to learn how easily I could manipulate pattern density by changing shell configurations, but the results of these changes are immediately obvious on the pattern board.
The little 16 ga. Parker bird gun I'm so fond of has odd bore and choke dimensions as I recall, but I can't remember the exact numbers sitting here at 3:30am. Regardless, it was averaging 58% (modified) patterns from the left barrel and 52% (*quarter choke) from the right at 40 yards with the load I was shooting. I recently switched to a different brand, dram equivalent, and wad type, so need to start the process all over and see how things have changed.
*Quarter choke, as I understand it and according to the chart I've adopted (from Jack O'Conner, I think), is of British origin and represents a choke having just a bit more constriction than our American improved cylinder. The chart goes something like this, according to notes I have scribbled here, and is calculated at forty yards:
Full Choke: 70% or above
Improved Modified: 65%
Modified: 55-60%
Skeet No. 2: 55-60%
Quarter Choke: 50%
Improved Cylinder: 45%
Skeet No. 1: 35-40%
Cylinder: 35-40%
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