I'm not sure how much of an effect they would have on suicides. This may just be my stereotypical mental image, but I suspect that a lot of gun suicide victims have owned the gun in question for a long time, in which case a waiting period wouldn't help any more than a waiting period to buy a car would stop people driving off bridges.

Are there any numbers on how many people commit suicide with guns they bought in the last 6 months of their life?