Much like this has already been argued, an M4's 14.5" barrel is plenty long enough to send a round out with accuracy and velocity for typical combat ranges. While a 20" barrel of say an FN FAL would shoot farther, it isn't so different that you could really tell. Also, a 5.56x45mm round has a velocity of ~3100fps, which is pushing Mach 1, so I don't know why you think it doesn't have good velocity, and that's just the standard grain bullet. 7.62 NATO is ~2,400-2,700 depending on loads.
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