Stun guns and rng

I'm not exaggerating and I am farming them. I've been doing 5 hard battles per day that purportedly drop stun guns, the other 5 hard battles I do do not have them. The rest of my standard energy goes to a 10-energy stun gun drop battle. I've hoarded all of my currency for the stores to buy stun guns if they appear, and they almost never do. I took intentional note of the timing because I last completed a stun gun on the same day I completed a zeta. And yes, since then I have completed 2 zetas and have only 45/50 towards a stun gun. Disagree with my conclusion if you like, but don't question my premise.