I have noticed that as well.. As you said, Efficiency is supposed to be killing more than one enemy with the Same bullet.. Not sure if this is just in my head, or if this is how the game is set up, but I've gotten more efficiency kills while using FMJ than when I don't.. And that is a good thing.. In the real world, bullets from a rifle are made to flatten and tear upon impact.. Which would make it very improbable for one bullet to kill more than one person.. However, a Full Metal Jacket round stays it's shape, and can pass through one thing and into another, making it very possible to kill more than one person at a time.. So, again, maybe I just feel like the game has it acting like real life, but maybe it's in my head..