New Zealanders had until Friday to surrender banned firearms under a mandatory government buyback after the country’s deadliest terrorist attack. But not all gun owners have heeded the call, raising questions about its effectiveness and offering lessons for gun-control advocates in the United States.

The Pacific nation outlawed semiautomatics and most high-powered military-style firearms after a gunman killed 51 people at two mosques in Christchurch on March 15. The trial of Brenton Tarrant, the alleged shooter, is set to begin in June.