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Yeah I have learnt a bit too. If you google it there's a lot of info.

Curiosity got the better of me and I had to know.

https://en.m.wikipedia.org/wiki/Caliber

I'm not sure but could be to do with barrel diameters, either inner or outer rifling measurement.

andyanimal31 is right about shot placement. For many years when I started hunting deer I just aimed at the chest area. As I gained experience I've learnt to pick the exact spot on the animal I want...

If you're researching buying one or the other the real option is to split the difference and get the mighty .270, the king of calibres for almost 100 years.