Yemen banned firearms two years ago, and the ban has not worked. While Yemen has a lower murder rate than the United States (4 per 100,000 population, compared to 6), that's four times the rate of neighboring Saudi Arabia. Most of the deaths are caused by firearms, which are particularly popular in the rural, and largely tribal, areas.
Yemen has long been a lawless, and heavily armed, part of the world. Until the discovery of oil to the north, particularly in Saudi Arabia, Yemen was the center of population (because of the annual monsoon rains), and wealth (because of Indian ocean trade routes) in the Arabian peninsula. But now most of the wealth, and population, is up north, where the oil is. But the lawlessness, and weapons, are still down south. Yemen has always been stirred up by tribal, ethnic and religious differences. The blood feud is an ancient tradition.
Thieves tend to be armed, and everyone feels a need to have a gun in the house for protection from theft, or worse. There are about a dozen weapons markets out in the countryside, and local authorities, who are often also tribal leaders, refuse to enforce the firearms ban there. This makes it easy for terrorists to get weapons, and not get in trouble while moving around with (usually concealed) weapons.
The new laws, and better policing (and more police) has reduced the use of firearms in the few cities, but out in the countryside, possessing a rifle or pistol is the rule.