Dustin picked out the vial labelled "Random Body Structure". He thought it sounded fascinating—he had always had a bit of a boner for the idea of a part of his body changing into something else. The endless possibilities and thoughts streamed through his mind: a clock for a face, knives for fingers, car tyres for feet, a power plug for a tail…
He considered the vial. Should he down the whole thing, or take only a little sip first? Having only a little bit of the serum might not do enough, but having too much might completely destroy who he is.
Deciding, Dustin picked to have a sip—maybe a sixth of the vial.
He took a little swig. The taste surprised him—it tasted like almost every food or drink he's ever had, good and bad.
He waited a little while to see what effects would come out of it. After about five minutes, he felt a wave of pleasure sweep his body suddenly, making him jump, centred on his dick. Excitedly, he took off all of his clothes to have a look at the changes he will be undergoing.
The first thing that occurred is that he feels his dick throbbing and going erect. He really wanted to masturbate, pulling back and forth on his cock, but he restrained himself as the changes went on. Eventually, it got too much for him, and he pulled back his foreskin and began to jack it off. He then felt the pleasure stop, even as he masturbated, and he took his hands off his dick. It was still very hard but not throbbing. He looked closer at it. Then, the foreskin retreated all of the way down his dick, leaving the inside standing there, which quickly grew gun barrel-straight and became steel, with a round hole at the top. Dustin realised that his dick had become a gun.
He was incredibly excited by this, and his gun-dick fired off a little shot into the ceiling with a deafening blast, him feeling extreme pleasure from it.