Actually this subject is very simple.
A .308 equals a 308 and is named that because it is 30 caliber.
A 30-06 is 30 caliber and was invented in 1906.
A .338-06 is a 33 caliber not invented in 1906.
A 30-40 is thirty caliber and was invented in 1892 (obviously).
A 38-55 has a wider bullet and more powder than a 30-40 and therefore is less powerful.
A 250-3000 goes 3000 feet per second.
A 30-378 doesn't go 378 feet per second.
A 6.5-54 uses a 6.5mm bullet and is 54mm long.
A 6.5-06 uses a 6.5mm bullet and is 2.494" long.
A 6.5-08 uses a 6.5mm bullet and is shorter than this.
A .308 bullet is the same as a 7.62mm bullet and is 7.92mm wide.
A 7.62mm NATO cartridge can be fired in a .308 chamber.
It can also be fired in a 30-06 chamber--exactly once! Shrapnel must then be removed from forehead.
410 gauge means .410" diameter.
Therefore 12 gauge means .735" diameter, of course.
