What is Calibre?

[BLUEHAWK]

:cd:

Okay, now you infantry are REALLY gonna make fun of me...

What exactly IS calibre? I mean, how is it measured, since there is also millimeter (mm) sizes that are talked about.

Are mm's and calibre the same? If so, then why not just use one or the other?

What is the smallest calibre or mm?

What is/was the largest ever? (I mean like Navy battleship 16 inchers etc)

Does EVERY possible shell have either a calibre or a mm?

When did these type measurements get started in history?

Is there any OTHER type measurement unit that is or ever was in use?

Did cannonballs have calibre or mm?

(I put this in here so you would not think I was afraid of guns)