I thought of this last night while I was falling asleep.
It seems to me that the best way to dispose of a murder weapon is to dismantle it, or reduce it to tiny pieces, then flush it down the toilet. Once it hits the sewers, it could go almost anywhere.
A very small knife, like the kind on a keychain, could easily kill someone then be flushed. The problem there is that knife wounds tend to gush and you have to worry about getting blood on yourself.
A handgun is neat and, if you have a screwdriver, can be dismantled and flushed. The problem with a gun is the noise, unless you have a silencer, but I think those are pretty rare. You could probably make your own with information from the Internet.
Is the search now for the perfect murder weapon? (Don’t worry, I’m not going to off someone. I’m supposed to be writing a short screenplay and this is the best idea I’ve had so far.)