To make things as simple as possible .308" is a bullet diameter or caliber.The bullet is the actual projectile while the assembly is called a cartridge.Many cartridges use the same caliber bullets but have different shell casings so they are named different.The difference in capacity and design of the shell casings are the reasons for different velocities.For example the following cartridges are all .308 caliber and use the same bullet
300savage
308win
30-06
300wsm
300h&h mag
308norma mag
300win mag
300ultramag
The same is true for the .284 caliber .These cartridges are .284 caliber and use the same .284 caliber bullet.
7x57
7mm-08
284win
280 rem
7mmrem mag
7mmwsm
7mmstw
I hope this gives you an idea as to the difference between a caliber and a cartridge.The caliber is the bullet diameter and all cartridges of equal caliber can use the same bullets.
grains is a measurement used to desribe the weight of the bullet and is used by reloaders to measure powder charges.


Edited by - stubblejumper on 12/29/2002 21:13:36