ok i dont really understand much about why a .410 is called a .410. it doesnt really matter to me but its just one of those things that just dont set right with me and im just trying to figure it out so if any1 has any info it would b greatly appreciated!!

SO FAR I UNDERSTAND :
- the .410 is a shotgun........but its a calliber not a gauge
- on some of my shells it says .410 AND 57mm

[:@]WHAT I DONT UNDERSTAND :
- it a friggin shotgun so why is its cal/gauge a CAL?! and in some casses a CAL AND A MM???!!![:@]
- why isnt it just a gauge?


Sincerely,
[&:]~~CONFUSED~~[&:]