Via email from Willi S.:
Participating in a gun buy back because you believe that the criminals have too many guns is like having yourself castrated because you believe that the neighbors have too many kids.
Willi doesn’t know the originator and a quick search revealed it’s been around for a while with no good attribution.
And on a more serious note don’t ever forget this one from Steve Levitt:
Gun buybacks are one of the most ineffectual public policies that have ever been invented in the history of mankind.