Don't give up hope though, once you learn some of the foundational stuff you can generally fill-in the gaps. Like the 30-06 and the 25-06...you can make a safe *guess* that it is a 30-06 (7.62X63) necked down to a .25 bullet. Also, a note of interest that has already been addressed: a .44 magnum actual shoots a .429 inch bullet and many other calibers only get "close" (You'll find that marketing has alot to with catridge names...if the competitors already make a 25 magnum...well then lets make a cartridge that shoots the same bullet and call it a a 250)
For a quick reference something that helped me mentally compare cartridges is to learn metric to standard:
8.60mm=.338
7.62mm=.308 or 7.62X51 (which is also all/most the .30 caliber bullets including all the 300 magnums)
7mm=.284 (or the .280...a necked down 30-06)
6.5mm=.260 (i think*)
6.16x51=.243
5.56X45=.223