CHRISTCHURCH, New Zealand — 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.

To get guns out of circulation, Prime Minister Jacinda Ardern turned to a method Australia implemented in the 1990s — buying back existing firearms while giving owners a grace period before their weapons would become illegal. Ardern's center-left government set aside about $110 million to compensate owners.

"We have moved to stop the sale, and now we've moved to stop the ongoing circulation of these weapons," she said in introducing the legislation.

The approach won wide public support and near-unanimous bipartisan backing in New Zealand, as well as praise from some quarters in the United States. Prominent politicians such as Sen. Bernie Sanders (I-Vt.), a Democratic presidential candidate, and former congressman Beto O'Rourke (D-Tex.) have proposed similar measures.

About 47,000 firearms have been collected, and about 2,000 others have been modified to become lawful, New Zealand police figures show.

A government-commissioned assessment by the accounting firm KPMG estimated that the number of banned guns could be between 50,000 and 170,000. If the median of that range were correct, more than half of the prohibited firearms would be unaccounted for; the precise figure is unknown because New Zealand until this year lacked a registry for military-style semiautomatics.

With the amnesty expiring, the nation's largest gun-rights group this week declared the buyback an "unmitigated failure," citing the group's research. Some two-thirds of weapons banned after the Christchurch massacre remain in the hands of New Zealanders, according to the Council of Licensed Firearms Owners, making those gun owners liable to five years' imprisonment.

The buyback "lacked fair and reasonable compensation for legally purchased items," said the group's secretary, Nicole McKee, who argued that Australia's buyback had incentivized gun owners by offering higher compensation for their weapons, as well as a longer time frame.

In New Zealand, compensation rates range from 95 percent for new or near-new weapons to 25 percent for those in poor condition. Some owners appear to be holding out for a better offer, though the government has ruled that out.

Officials are hailing the policy as a success. "I just do not believe there's 170,000. I believe we've got the majority of these guns in," Police Minister Stuart Nash told New Zealand's national broadcaster on Tuesday.

Advocacy group Gun Control NZ stressed that the buyback was only one aspect of the government's wider policy response, along with tougher licensing, registration and stronger enforcement powers.

Nik Green, the group's co-founder, said it was difficult at this point to form a definitive judgment on the success of the buyback.

"On the one hand, taking around 50,000 of these weapons out of the community is a clearly positive step," she said. "On the other hand, we don't know what proportion of all prohibited firearms this represents. If we use the lower-bound estimates, it's a pretty good result; if we use the higher ones, it's less so."

Whatever the outcome, it's doubtful New Zealand's approach would be feasible in the United States, said Eugene Volokh, a conservative blogger and legal scholar at the University of California at Los Angeles.

"The great majority of American rifles and handguns are semiautomatic. I can't imagine American gun owners going along with this, even to the modest level of compliance that New Zealand seems to have gotten," he said.

New Zealand's gun culture differs from that of the United States, where the right to bear arms is enshrined in the Constitution's Second Amendment. While firearms ownership is not unusual in rural communities for hunting or culling purposes in New Zealand, it is rare to see a weapon in an urban home.

Police estimate that legally owned firearms in New Zealand numbered up to 1.2 million in 2014, broadly in line with a more recent study by the Small Arms Survey, run by the Graduate Institute of International and Development Studies in Geneva. That equates to about 26 civilian-held firearms for every 100 people — far lower than the United States but almost twice that of Australia, whose buyback resulted in the surrender of more than 600,000 weapons.

In New Zealand, lobbyists on both sides of the debate have called for an extension of the amnesty period to allow for more guns to be collected. But authorities have declined.

Polls show the public strongly supports gun control. Nonetheless, the buyback has drawn pushback from a modestly sized but vocal protest movement, spurred by frustration over compensation rates, the speed of the legislative changes, late adjustments to the list of banned weapons and even a data breach on an online registry.

Their concerns were heightened by the introduction in October of armed police patrols in New Zealand cities, which have already featured in two fatal encounters.

Travis Poulson, a 38-year-old hunting enthusiast from Auckland, said there was resentment toward the police. He added that he believes the Christchurch mosque attacks could have been prevented if the alleged attacker's gun license had been subject to more stringent checks.

"It is the public that are paying the price through no fault of their own," he said.