Assuming 1/9 dropchance on any of the weapons, the probability to get minimum 1 drop is x=1-(8/9)^n where n is the number of kills and x is the probability:
n=1 > x~11%
n=3 > x~30%
n=6 > x~50%
n=10 > x~70%
n=20 > x~90 %
n=50 > x~99.7%
n=100 > There would be a 1 to 130000 for not getting a drop, which is still quite probable compared to winning the lottery.