Ballistic coefficient and caliber are not as directly related as some people would think. The ballistic coefficient of a bullet tells you how well a bullet can overcome drag and other atmospheric resistance, and that is about it. I am not sure why the military has gone with the 30 caliber cartridges instead of the 7mm or 284 calibers.
The most probable reason could be because the roots of the 30 caliber military rifle cartridges go all the way back to the 30-40 Krag and Teddy Roosevelt. For me it is kind of like asking why the V-8 motor is still so popular when different variations of 6 cylinder and 4 cylinder motors can do so many good things. The V-8 was kicking the crap out of everything 50 and 60 years ago. All the new motor variations have come out to compete with the V-8 not being able to replace it. So the V-8 is the standard. I believe the same is true for the 30 caliber military cartridges of WW-1, WW-2, Korea, and Vietnam. Nothing was better, more reliable and more lethal than the 30 caliber service rifles. So that became the standard and all the new weapons were built around that caliber and the cartridges that fired it. You may ask what about the .223 or the 5.56 nato. Like I said, better, reliable and lethal. Ask the men who carried both and ask them which they prefer. The overwhelming majority will tell you, when it comes to lethality in any situation, the 7.62x51 or 308 Win is the one. I think the Ballistic Coefficient has evolved based on demand not an "inherent superiority" of the 30 caliber bullet.
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