Atta-boy, Notchy. Good to see someone actually reading books and articles rather than just relying on the internet.
Would seem that back in the day the 28 and 24 Ga were the most used hunting smoothbores both for Settlers and Indians.
Ferret Master, some of the mid-20th century gun writers and historians did our hobby a great disservice with their mixing of antique and modern terms. Prior to the middle of the 19th century, the common way of referring to a gun's size was in the number of balls to the pound that it shot. In period documents this was often written as "28 to the pound" for English guns or "28 to the
livre" for French guns. The
livre being the French unit for weight. Modern gun writers tired of writing out "to the pound" and chose to use the modern term "gauge" instead.
The table that
Notchy Bob shows above is correct for the bore ID of a modern shotgun. It is also correct for the OD of a lead round ball, the number of which would weigh a pound. But a gun that would shoot a ball 28 to the pound does not have the same bore ID as a 28 gauge shotgun since that would mean the ball OD would be the same as the bore ID. That would be very difficult to load in a muzzleloader, especially with a patch and after the barrel accumulates some fouling. There needs to be some space for the patching and the buildup of fouling. As Notchy Bob said, "The difference between ball diameter and bore diameter is termed 'windage'."
I've read several writers quote a period document that should say "28 balls to the pound" with "28 gauge". From Notchy's table above, a ball 28 to the pound would have an OD of 0.550", and add to this a windage of approximately 0.03", you have a gun with a bore of 0.58". A modern 28 gauge shotgun has a bore of 0.550". See the confusion. A modern barrel of 24 gauge (0.579") is closest to a gun that carries 28 to the pound (0.550"), assuming a windage of just under 0.03".
In the day, smooth bores tended to have more windage than rifles. A smoothbore musket could have windage around 0.05" while trade guns seem to have windage more on the order of 0.03". The windage for rifles back then was closer to 0.01", similar to today. Of course, manufacturing tolerances back then weren't as tight as they are now, so balls-to-the-pound and windage were nominal figures.
Balls to the Pound Equivalents to Modern Shotgun Gauges for Smoothbores
24 bttp = 0.579" + .03" windage = 0.61" bore or close to modern
20 gauge
28 bttp = 0.550" + .03" windage = 0.58" bore or close to modern
24 gauge
32 bttp = 0.526" + .03" windage = 0.56" bore or close to modern
28 gauge
Balls to the Pound Equivalents to Modern Calibers for Rifles
32 bttp = 0.526" + .01" windage = 0.54" bore or close to modern
.54 caliber
40 bttp = 0.488" + .01" windage = 0.50" bore or close to modern
.50 caliber
60 bttp = 0.427" + .01" windage = 0.44" bore or close to modern
.44 caliber
The best systematic study of existing Northwest guns (that I know of) is Ryan Gale's For Trade and Treaty. Most of the Northwest guns in that book run around .60 caliber, so although they may be described as 24 gauge, they are actually closer to 22 gauge.
Notchy is correct regards to modern shotgun gauge, but in 1855, the British government required the bore size in balls-to-the-pound be stamped on the barrel along with the proof marks. If you look at the guns starting on pages 41 and 47 in Gale's
For Trade and Treaty, they are dated after 1855 and have "24" stamped on their barrels. Gale lists their bores just below and just above 0.60". These nominal 24 ball-to-the-pound guns are equivalent to a modern 20 gauge once you allow for windage. Starting on page 53 of the same book is a double gun dated 1904 that is stamped "27" while the bore size is given as 0.568". Apparently by this late date, manufacturing tolerances were tighter and the windage was considered to be closer to 0.01" since a ball 27 to the pound has an OD of 0.557".
