" A block of wood with several holes bored to the correct size..." is the critical observation that give you the answer.
Tools were rare and expensive. Today we pay a couple of dollars for a drill bit, and a drill or brace, and grab a piece of wood from a home depot, or wood supplier, and we can drill away to our hearts content. Early drills were spoon bits, hard to forge, and slow to use. They were used in the timber framing business, and the spoon bits were an inch or larger. If you could find one that was only 1/2 " in diameter, it would cost you much money. Money is something that was always in short supply in the colonies. So, you didn't have ball blocks, or blocks to hold paper cartridges in a leather kit or box, as the soldiers did. Because calibers were totally dependent on the whim of the barrel maker, who also then made a cherry and cut a mold for the ball for that barrel, no one thought of making a uniform sized cartridge box for people to carry " cartridges " in. And paper was also in short supply. That is why powder was carried in horns, and patching could be made from small animal skins, or old cloth, or even using leaves in a pinch. Up until the 19th century, gun barrels were made to a " gauge" and sold as a gun that shoots X number of ball to the pound. Caliber meant nothing to people who had no tools to measure calibers! But everyone could count balls to the pound!
A 75 caliber musket ( brown bess) was an 11 gauge gun. A. .69 caliber is a 14 gauge. A .58 cal. is a 24 gauge. A 54. cal. is actually a 29 gauge. .50 is a 37 gauge. .45 is a 51 gauge. .40 is a 72 gauge. A .36 is a 100 gauge. and a .32 is a 142 gauge gun.
The smaller gauge rifles were very expensive to make, because the boring bits had to be made so small, and were equally fragile. The cost of making the tooling to build such a small bore gun was taken into consideration in pricing the gun. Smoothbore shotguns and fowlers were popular throughout the 17th and 18th centuries because they could be used to take both birds, and small mammals using shot. The guns cost much less to produce and to own than a small bore rifle.
It really was not until the Industrial age allowed mass production that small caliber rifles became available in any quantity for civilian use. We are talking the age of BP cartridges here, beginning with the .22 rimfire (short) cartridge in 1857, and others that came out as centerfire cartridges in the 1870s, and '80s.( the .38-40, the .32-20, and the .25-20 come to mind.) ( There are some references that refer to the .45-70 rifle as a " Needle gun "!)
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