So many people cannot get that one last raid weapon. I hear someone in every normal mode raid I do, missing just one raid weapon. For me it's Black Hammer. In 21 NM completions I have gotten 2-3 of all the other special/heavy weapons, but no Black Hammer. I am starting to think Bungie is tweaking the drop rates dynamically to reduce your chances of getting that last remaining weapon. Reason: to keep you playing longer, and/or come back to the game to try again, i.e. keep the hamster wheel spinning.
Or: a lot of people are just really, really unlucky.
Update: 18 more completions, still no Black Hammer (zero weapons three weeks in a row). I have now completed NM 39 times, I have received 5 Swordbreakers, 4 Light of the Abyss, 4 Hunger of Crota, 3 Song of Ir Yut, zero Black Hammers.
Based on my drops so far, if I figure there's about a 60% chance of getting a weapon from Crota NM, and there are 5 weapons, Light, Sword, Song, Hunger, BH, if RNG is fair, there's about a 60/5 = 12% chance of getting BH. There's an 88% chance of not getting it. So to _not_ get it 39 times in a row has a probability of 0.88^39 = 0.0068 (0.68%) - or one half out of 100. That's pretty bad luck! This assumes independent, completely random events, which may or may not be true. And the 60% guess is based on way too few results to be anywhere near accurate. But given those two assumptions, one can then calculate the probability of the series of events of not getting a drop, and plug in various numbers to see how it affects the probability of the whole series.
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Everytime you finish it rolls the "get a weapon" dice. Let's call it 60% you get it. Then it rolls the "which weapon" dice. Every single roll you have a 12% chance by your math to get that specific weapon. I know each time you flip a coin, the odds are always the same. Over 100 flips, it should even out. But it's entirely possibly to flip talks 100 times in a row...