Are Rifle Calibers Getting Smaller?

In the last 50 years or so, there have been several arguments about the 5.56x45 mm. cartridge and its small bullet. Some argue that the larger bore 7.62x51 mm. cartridge is harder hitting and therefore better. Others argue that the 5.56x45 mm. cartridge is lighter, but hits adequately, therefore a person can carry more of them. This caliber debate has been going on for a while. Bear in mind that in the early 1950s, when the 7.62x51 mm. cartridge was first introduced, several people from that era thought that *it* was a smaller cartridge! This is because it replaced the larger .30-06 cartridge which was in service since about 1906. As we will soon see, the decrease in size of cartridge calibers has actually been going on for a lot longer.

In the early part of the 19th century, soldiers mounted on horses (cavalry) were still an important part of many armies. We hear accounts of several famous cavalry battles, such as the Charge of the Light Brigade (and the lesser known Charge of the Heavy Brigade at the same battle),  Pickett's charge, Battle of Little Bighorn etc. It was the opinion of military experts of that period, that the bore of an infantry musket must be large and the bullet heavy enough, to stop a charging cavalry soldier. It was believed at that time that a smaller bullet, even with greater velocity and equal momentum  compared to a larger bullet, would only wound the foe, but not instantly disable him. However, it was later found by experiment, that the increase in velocity of a bullet makes up for what it loses in mass, and a lighter bullet has greater range and a soldier can carry more of them, which makes the infantry man much more effective in the field. Therefore, since about 1850, as firearm technology gradually started moving towards rifles, the size of bullets have been decreasing with every advance in infantry weapon technology. The following table is largely transcribed from The Gun and its Development by W.W. Greener and lists the diameters of bullets from various military forces in Europe and America from 1850.

Year	Country	Firearm	Caliber
1850	England	Brown Bess (11 bore)	.750 inch (19.2 mm.)
1850	England	Brown Bess (14 bore)	.693 inch (17.85 mm.)
1852	England	Enfield	.577 inch (14.8 mm.)
1854	Austria	28-bore rifle	.550 inch (13.8 mm.)
1860	Sweden	40-bore rifle	.488 inch (12.6 mm.)
1866	France	59-bore rifle	.433 inch (11.0 mm.)
1867	Austria	62-bore rifle	.420 inch (10.7 mm.)
1869	Switzerland	75-bore rifle	.400 inch (10.4 mm.)
1871	Germany, Spain and Holland	58-bore rifle	.433 inch (11.0 mm.)
1871	England	51-bore rifle	.450 inch (11.43 mm.)
1874	France	58-bore rifle	.433 inch (11.0 mm.)
1878	Sweden	76-bore rifle	.396 inch (10.15 mm.)
1880	Serbia	76-bore rifle	.396 inch (10.15 mm.)
1886	France and Portugal	150-bore rifle	.315 inch (8.0 mm.)
1887	Turkey		.350 inch (9.5 mm.)
1887	England	Enfield Martini	.400 inch (10.25 mm.)
1888	Germany	156-bore rifle	.311 inch (7.9 mm.)
1888	Germany	150-bore rifle	.315 inch (8.0 mm.)
1889	England	172-bore rifle	.303 inch (7.7 mm.)
1889	Belgium	173-bore rifle	.303 inch (7.65 mm.)
1889	Denmark	150-bore rifle	.315 inch (8.0 mm.)
1891	Switzerland		.295 inch (7.5 mm.)
1891	Italy		.256 inch (6.5 mm.)
1891	Russia		.300 inch (7.62 mm.)
1892	Spain		.276 inch (7.0 mm.)
1892	Holland and Romania		.256 inch (6.5 mm.)
1893	USA		.300 inch (7.62 mm.)
1895	USA (US Navy only)		.236 inch (5.87 mm.)

As can be seen in the table, the diameter and size of the bullets has been decreasing for quite a while. As propellants improved and black-powder began to be replaced by more powerful smokeless powders, the sizes and weights of the bullets began to decrease as well.