Why the use of strange calibers like 5.56mm, 7.62mm, 155mm, 203mm?

Its a tragedy thats the .280 British never saw service. It had much more ooomoph than the 5.56mm but less of the full auto controllability issues the 7.62mm did.

I just want to point out that the the two most popular 7.62mm rounds, the 7.62x51 NATO and the 7.62x39 Kalashnikov bullets are actually 0.308 and 0.311 inches in diameter.

And now the US is moving towards adopting 6.8x51mm/.277Fury.

You know, I Googled that round, and while I found plenty of information about PSI and other ballistic data, I can’t fine anywhere that says how much it weighs. How many rounds fit in a magazine? How many mags can you carry? You know, the important stuff.