What does the .06 mean in 30.06 caliber?

The ".06" in the 30.06 caliber refers to the cartridge's case length, which is 3.06 inches. The "30" indicates the bullet diameter, which is .30 inches (or 7.62 mm). The .30-06 Springfield cartridge was introduced in 1906, hence the designation "06." It is widely used for hunting and military applications due to its versatility and effectiveness.