Here's some trivia. The US Govt. has adopted 3 .30 caliber rounds over the years, 30/40 Krag .30-06 and 7.62X51. Everytime they have used a different method of designation. .30/40 Krag, was designed as the last black powder cartridge, .30 caliber 40 grains of powder for the US Krag Jorgenson rifle. .30 -06 (actually there was a short lived .30-03 as well) .30 caliber adopted in 1906. Then of course the 7.62X51 is the metric measurement. According to my Sierra reloading manual, (I checked) all use the same diameter bullet, .308.