Ed brings up several very good points. Even the proof houses eventually recognized there was variation between batches of black powder. Eventually tools like this were developed so they could measure the difference.
The early workman did not have the most precise instruments available. For a long time only rough gauges were available to them.
Add to this that the measure of a bore depended upon the pound. A pound was not the same every where at all times. There was a variation that occurred from country to country.
Pete
