It hasn't been proven to be effective where all guns are illegal. For example, Mexico, where there is a large problem with drug cartels and their weapons. Most law abiding citizens will turn in their guns in the event of a weapons ban, but the criminals will have a much easier time of getting their "job" done.
Nations with stringent anti-gun laws generally have substantially higher murder rates than those that do not. The study found that the nine European nations with the lowest rates of gun ownership (5,000 or fewer guns per 100,000 population) have a combined murder rate three times higher than that of the nine nations with the highest rates of gun ownership (at least 15,000 guns per 100,000 population).
For example, Norway has the highest rate of gun ownership in Western Europe, yet possesses the lowest murder rate. In contrast, Holland's murder rate is nearly the worst, despite having the lowest gun ownership rate in Western Europe. Sweden and Denmark are two more examples of nations with high murder rates but few guns. As the study's authors write in the report:
If the mantra "more guns equal more death and fewer guns equal less death" were true, broad cross-national comparisons should show that nations with higher gun ownership per capita consistently have more death. Nations with higher gun ownership rates, however, do not have higher murder or suicide rates than those with lower gun ownership. Indeed many high gun ownership nations have much lower murder rates. (p. 661, Harvard Journal of Law & Public Policy Vol. 30, Num. 2)