Of course you can try to start somewhere and the measures mentioned in passing by olddude would be a good start - how that goes about changing the perceived notion that one has to be capable of defending oneself against your own government and the mistrust and belief that your own government is going to attack you is going to be a "North face of the Eiger" type climb in a country riven by schisms, some of whom date back to it's conception.

So while you may make a start, realise the scale and magnitude of the problem (And that is only taking registered firearms into account) and resign yourselves to the fact that this sort of atrocity will be repeated in the future time and time again as if ever there was one - this is most definitely a case of shutting the stable door after the horse has bolted.