i think you people are under the illusion that the government got all the firearms in 1996? see most people dont go and kill others with firearms, the government does that (through army, police etc), most pre 96 firearms are still out there, then add imports (they check 1/100 containers), firearms made here etc.
for example norinco imported 600,000 sks (chinese ak-47) in the early 90's and only about 80,000 were handed back in, these sold for $150 in a wooden crate with ammo, another example is in nz no guns are registered and you can buy anything from an ak-47 to an uzi, and where are the mass shootings? they dont exist