Gun buybacks are pretty useless. Sometimes pro-gun activists will turn in thirty dollar guns for the hundred dollar credit, and tell the press they're investing them in a Glock. Libertarian Steve Chapman notes  the uselessness, and mentions the demographic aspect:

Think about it: Who is most likely to turn in a firearm for a $100 reward? Someone with 1) a cheap gun and 2) no criminal propensity—say, Aunt Millie disposing of a rusty revolver her late husband left in the nightstand.

Florida State University criminologist Gary Kleck noted in a 1996 book that when St. Louis did a gun buyback, those participating "were commonly middle-aged and 80 percent white, while those involved in gun violence in that city were mostly young and black."

The Futility of Chicago's Gun Turn-Ins | Who is most likely to turn in a firearm for a $100 reward? Someone with 1) a cheap gun and 2) no criminal propensity.By Steve Chapman, Reason Magazine, June 18, 2012

The demographic insight is Kleck's—previous Chapman items here showed him denying the demographic aspect of crime.