Originally Posted by
Rongway
So, if there are fifteen weapons each with an equal probability of dropping, there is a (1 - (14/15)^n) probability that you will see the weapon you want within a given set of n runs.
Now, given a high enough n, this value approaches, but does not reach 100%. And remember what I said earlier: if the probability is not 0% or 100% you cannot claim any guarantees.
Does it suck that you haven't seen the item you want after X runs? Sure. Is something wrong with the system? No. You just don't understand randomness.
Even if the one weapon you wanted had a 99% chance of dropping, there still exists a 1% chance that you will not see it after a given run. After 100 runs, there would be a 99.9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999% chance that you would see the item you want. But this still isn't 100%, and though it would be incredibly unlikely that you didn't see the item you wanted, it would still be possible.