To study that topic, we must go back to a time when bullets were cast round lead balls. The bore or gauge was determined from the weight of a solid ball of lead that could fit into the barrel. For instance, if a spherical ball weighing 1/10th of a pound could fit into the barrel, then the gun was considered to be 10 bore (or 10 gauge). If a ball weighing 1/12th of a pound could fit into the barrel, then it was 12 bore (or 12 gauge) and so on. Conversely, if you had 1 pound of lead available, you could cast 12 bullets for a 12 bore (or 12 gauge) gun and 10 bullets for a 10 bore (or 10 gauge) gun and so on. Even though bullets were no longer spherical shaped, this unit continued to be used into the 19th century to denote the barrel diameter, assuming that a spherical ball of lead was used.
As you might have gathered from the paragraph above, the smaller the bore number, the larger the diameter of the barrel. Therefore, a 4 bore gun would have a large diameter, since 1 pound of lead would only make 4 solid spherical bullets for this gun. As you might have guessed, 4 bore guns were called "Elephant Guns" and used to shoot large animals like elephants, buffaloes and rhinos.
In the picture above, we have 3 cartridges, left to right: a .30-06, an eight bore and a four bore cartridge. Note that the .30-06 is the cartridge used by the M1 Garand rifle and measures 7.62 x 63 mm. This gives the reader an idea of the size of one of these large bore bullets.
By the way, the four bore wasn't the largest elephant gun around. There was also the two bore. The two bore was only used by very few men because of its immense recoil and weight.
The above picture shows a .700 Nitro express bullet and cartridge case versus a 2 bore bullet and cartridge case. The diameter of a 2 bore is 33.68 mm and the bullet weighs over 225 grams! The famous British explorer, Sir Samuel White Baker, owned one such weapon and he was afraid to fire it. In his own words:
The two bore was the largest rifle that could be fired from the shoulder. There are still larger calibers though. One of these examples is the 1 bore or punt gun, which was designed to be fired from a boat. We will discuss punt guns in a future post.