system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the muzzle energy of a bullet is = 1/2 * M * V^2, wouldn't it make sense to make really small and fast bullets since it has a much greater affect on energy than mass does?

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gpt
Your intuition is correct, and that is exactly how cartridge selection for the military has progressed over the years. 

The standard issue rifle for US troops, the M16/M4 fires the 5.56 Nato cartridge, which has a very small (0.15 ounce) bullet traveling at blistering speeds (2000mph!)

Compare that to the previous US military infantry rifle catridges that had bullets that weighed 3 times as much like the 30-06 in the M1 Garand.  

There are other factors that went into selecting a catridge with a lighter, faster bullet (accuracy during full auto, amount of ammo a soldier can carry, politics, et cetera) but militaries around the world tend to favor light, very fast bullets for infantry rifles.