The .308 Winchester/7.62x51 is a modified .300 Savage.
The Army thought the .300 would make a cartridge with near the power of the .30-06, in a more compact package.
In testing, they found the neck of the .300 Savage was too short for military use in semi-autos/full autos, so the increased the case length a bit and increased the neck length. Voila! the .308/7.62x51.
My understanding is the .223/5.56x45 is the result of similar experimentation with the .222 Remington magnum.
I just looked and found that I have darn near a full box of the old Bronze Tip ammunition in .300 Savage. Damn shame I dont have the rifle to go with it.
Fine old lever gun that 99. The rotary magazine was an interesting feature.
L