@jeck The M-4 has a 14.5" barrel, the Mk.18 has a 10.5" barrel. The result is the Mk.18 fires bullets at just barely above the velocity where 5.56mm stops being effective. The result is that the M-4 is a fair bit more powerful.
Still, half a mag to down someone is pretty excessive, I've not noticed that myself, but I do notice the Mk.18 takes a few more shots to take someone down than the M-4. Them killing you while you shoot them really needs to be changed. That annoys me a lot too. Bots especially should not be able to one-shot me in the head as I'm dumping rounds into their chest.