RE: kid question
The system has no rhyme nor reason to it. Some rounds use mm others dont some rounds use the year they were introduced '06 others don't. Some use a reference back to a parent cartridge. Some refer to the amount of black powder used others dont.The best way to teach him is to show him different rounds and let him compare. Also, let him know that there is no constant to it so he doesn't try to make since of it. You just have to work your way through the different rounds. Also get a copy of Cartridges of the World it will help compare.