New Zealand is trying to buy back the military-style weapons it banned in April. It’s not so easy.

Growing opposition from New Zealand’s pro-gun groups has complicated efforts to round up the now-banned firearms under a buyback program. Lawsuits are threatened.

Gun-control advocates argue that compensation rates may not be fair and warn of a possible spike in black-market sales.

The government, meanwhile, is faced with a sobering set of challenges over how to enforce the new law.

There is no national registry for many of the weapons targeted by the ban, including the AR-15 — a semiautomatic rifle that has been used in mass shootings in the United States and is often at the center of American gun-control debates.

So far, about 700 firearms have been voluntarily surrendered. The authorities, who estimate there are 1.2 million to 1.5 million firearms in New Zealand, have written to every registered firearms owner to inform them of their obligations.

Paul Clark, owner of New Zealand Ammunition, one of the country’s largest ammunition companies, said he believed many owners would attempt to hide their weapons.

He added that if owners are not allowed to make their case through the justice system “the only alternative is revolution.”

Asked by Radio New Zealand journalist Lisa Owen to clarify what he meant, he replied: “Literally, what I just said.”