I didnt read all the replies so someone may have already said this.
A "caliber" is 1/100th of an inch and is a unit of measurement. so a .25 caliber bullet is 1/4 of an inch and a .50 caliber bullet is 1/2 of an inch and a 1.00 caliber would be one inch. However calibers in cartridge NAMES are usually rounded like the .38 Special actually uses a .357 diameter (caliber) bullet. "30 caliber" generally means a cartridge plugged with .308 caliber bullet. Whether it be a .300 Weatherby or a .30 carbine .308 is usually the ACTUAL bullet diameter. Almost all calibers are rounded to some extent, like .270 Winchester is actually .277. .260 Remington is actually .264. Some are more truthful like the .284 Winchester or the .500 S&W. I saw some good replies on the history and origin of both the 30-06 and .30 carbine so I'll not delve into that. I sure hope this doesnt make things more complicated! Welcome to the rifle world.