From Politifact:

A study done by one researcher from Georgetown University and one from Duke University that was published in the Journal of the American Medical Association in 2000 examined the Brady Handgun Violence Prevention Act, a 1994 federal law that established a nationwide waiting period and background check for handgun sales. (The waiting period provision was later removed.)

The study concluded that the law’s waiting period was associated with reductions in the firearm suicide rate for people age 55 and older, but not associated with reductions in homicide rates or overall suicide rates.