Sorry, but you can't look at a knife wound and say "Oh, well, this was cause by a (insert brand here) knife, and it matches the knife wounds caused in these other victims. people can match up bullets to victims which makes for definitive evidence in passing sentence on crimes. There's also the fact that guns leave DEFNITE traces when someone has just used them. guns are LOUD. Even with earplugs it'll still leave your ears ringing to fire them, just pulling one out and unloading is going to leave you nearly deaf for a while not to mention alreting EVERYONE in the nearby area that something is up, and the gunpowder leaves a VERY distinct taste in the air as well.
to a certain extent, they can make guesses if you have a string of identical knife wounds, but usually only because knives are so rarely used in favor of guns, if you had more knives being used, it'd all just look the same.
It sorta makes me wonder, really. How many violent crimes are there that aren't gun related that simply are overlooked because no crime can be proven? I mean, people cut themselves on things, fall down and hit their head off of a wall or a door. when you have knife wounds and basic blunt trauma, there's really not that much difference, some people might not report it, some people might credit these incidents to self inflicted or accidental injuries. A bullet wound is highly identifiable though, and you don't just bump against wall and suddenly there's a bullet in your shoulder, someone had to have fired a gun in order ot put it there, so pretty much every incidence in which a gun is used is going to be identified, while other violent crimes are much mroe easily overlooked.
Currently wrapped up in: Half Life, Portal, and User Created Source Mods