The tweet drew a barrage of derogatory messages to his Facebook account – most apparently from the US, where the pro-gun lobby is powerful and vociferous.
Mr Hart deleted the messages but posted online: “A warm kia ora to all my new American Facebook friends.”
“I’m not familiar with your local customs, but I assume ’Cuck’ is a traditional greeting,” he said of the insult, short for “cuckold” frequently used by far-right pundits.
Police said they did not have data available on the number of weapons handed in since Friday.
But they issued a statement saying that “due to heightened security and the current environment, we would ask that people please call us first before attempting to surrender a firearm.”
Ms Ardern has said that details of the government’s proposed law changes on gun ownership will be announced by next week, but she indicated that gun buybacks and a ban on some semi-automatic rifles were under consideration.
“As the Cabinet, we were absolutely unified and very clear: the terror attack in Christchurch on Friday was the worst act of terrorism on our shores, it was in fact one of the worst globally in recent times, it has exposed a range of weaknesses in New Zealand’s gun laws,” she said.