Firearms nomenclature is a confusing, often contradictory mess. No single systematic means were adopted to designate cartridges or weapons and if varies from both country to country but internally from manufacturer to manufacturer.
Brown Bess Musket
In the early days most weapons were designated by bore diameter or by designer or manufacturer. Take for instance the Brown Bess, nobody knows the origin of the nickname for the officially named Land Pattern Musket. This was the British firearm of choice during the Revolutionary War. First the term musket. The musket is a long arm (two handed weapons fired from the shoulder) muzzle loading originally flintlock smoothbore weapon. It was a large bore weapon firing .75 or ¾ inch projectile.
Black Powder & Round Lead Balls
When black powder was the only propellant available, velocities were limited to under about 1,800 feet per second (fps), the only way to increase power was to shoot a bigger ball. Muskets were limited to round ball. Black powder produces a lot fouling or solid deposits in the weapon. Only about 40% of the powder charge is converted to expanding gas to push the projectile. All that smoke is solid particles expelled from the muzzle. A lot is left behind in the gun. As shooting commences in combat more carbon and other materials build up in the barrel. Because of this the projectile needs to be smaller than the bore. Thus, as the lead ball clatters from side to side or up and down the barrel and the ball will exit at an angle to the bore and aim point. Because of this, accuracy on man sized targets is limited to a range of about 40 yards range. However, getting hit by a ¾ inch ball of lead at about a 1,200 fps will definitely ruin your day, particularly if you take a strike on bone in your extremities, thus the popular remedy was amputation, mostly it amounted to just cleaning up the stump. Former colonial groups of Americans used the Brown Bess as well. The Americans preferred the ball and shot loading them with the .75” ball over a load of three buckshot, ensuring the possibility of a hit.
Musket Paper Cartridges
The ball reigned supreme as the projectile of choice in both muskets and rifles (originally designated as the Rifled Musket or Musket Rifle, later as just a rifle.) Muskets however used paper cartridges. Instead of a powder horn or flask, the ball was wrapped into paper with the charge. Thus, fixed charges were used based on the weight of the charge. We get the beginnings of cartridge nomenclature. The paper cartridges where made by wrapping a string around a paper tube at each end and another third string wrapped to separate the bullet from the powder. You bit the powder end off, pored a bit in the priming pan. After closing the pan, you poured the powder down the barrel followed by the paper wrapped ball, paper first that acted as the wadding to seal the chamber and prevent the ball from rolling out the barrel. The point is that it provided a fixed charge.