People who don't like guns want to prevent people from owning them. Those people usually go for the low hanging fruit that appeals to most people like preventing the mentally ill to buy firearms. However it is important to note some unpleasant facts like the fact that 11% of Americans are on some antidepressant and probably many more were on some anti depression medication in the past [0]. Preventing access to those seemed mentally unfit (by some standards) would prevent a significant portion of the us population from owning a firearm. Whether that's right or wrong, I can't say.
http://www.medicaldaily.com/antidepressants-arent-taken-depr...