and then you take the 30-06... 30 caliber (.308) and invented and put into service in 1906... yes it can be very confusing if you've never taken the time to investigate it, etc... then folks started necking the 06 up and down and you ended up (and bumping shoulders so you can't load the wrong round in the wrong gun) with the likes of the 25-06, 270 Winchester, 280 Remington (also know as 7mm Remington Express), 338-06, 35 Whelen, 375 Whelen, 400 Whelen and probably a lot of others...
you really want to be confused... take 38 special and 357 magnum... you can fire both from a gun chambered for 357 mag but the bullet diameter is actually .357 or .358 (yes they do vary depending upon manufacturer and what not)... and the 44 mag bullet is actually .429...

reloading manuals have short write ups about just about every common round and blueprints of the cartridge along with loading data... invaluable to many people... i highly recommend spending the 30 or 40 bucks...