without a doubt, "skin-popping" is the most unsettling slang term for shooting heroin i've come across. (and if you've gotten certain body piercings, or been around them being done, you know that it does pop and ohgod.)
haha! i don't think it's named for the sound, just the action of puncturing. like.. popping a pimple? (*hork,* indeed!)
it's only in tough skin, and only on the way out, not the way in. (almost a tiny version of the (echo-like?) sound of uncorking a bottle of wine.. piercing needles are hollow, to thread the jewelry in behind them). but, in any case, tattoo machines have noisy vibrating parts (in the same way sewing machines do), so no one knows what noises the skin could be making!