Gun City boss David Tipple has told MPs the ban on semi-automatics "ignored what went wrong" in how the alleged Christchurch gunman obtained firearms and ammunition.
He estimated that the firearms buyback scheme would cost more than $700m - not the up to $200m the Government expects.
Tipple reportedly called journalists at Newshub "terrorists" before appearing before the finance and expenditure committee the committee.
MPs, including Police Minister Stuart Nash, said afterwards that was not becoming behaviour from Tipple.
The committee is hearing oral submissions on the Government's gun law reform bill, which would ban military-style semi-automatics (MSSAs) and assault rifles and related components, with some strict exemptions. The law is being reformed after the Christchurch mosques shootings that killed 50 people and injured 40 others.
Tipple said the mosque tragedy has created an unprecedented unity throughout our country - but this compassion should be extended to law-abiding gun owners.
Tipple said the bill ignored what went wrong and there were no loopholes in existing law. The mosque gunman had broken the law by using a 30-round magazine, he said.
"Understandably, all of us in New Zealand cry out for a solution."
The alleged gunman had wanted his actions to restrict all kinds of firearms to law-abiding citizens. Passing the bill would help the gunman win, Tipple said.
"His aim was division, intolerance and hatred, and he was using a firearms debate to get his way."
Tipple said large magazines can be prevented from fitting an A-category gun.
This would the most effective solution and would half the money needed for compensation - money that could go towards many other worthy projects, such as child poverty.
Tipple said E-category restrictions were tight and needed no further restrictions.
"What's wrong and unjust about the proposed law? It hasn't addressed how he got his licence. It takes away rights that don't need to be taken ... There are livelihoods at stake."
He said suppliers may not allow dealers to return MSSAs and assault rifles and their parts, because many of the guns were specially made and will not be wanted overseas.
"In most cases, it won't be possible to export the gun for parts."
Tipple said more time was needed to consider the bill.
He said he had gone through Gun City sales and estimated a buyback scheme would cost $726 million. But doing as he suggested would reduce that to $315 million.
The numbers came from company sales data across New Zealand on each type of firearm that the bill would ban.
"Our strength and our unity is because we accept differences and we tolerate them. Passing this law means the murderer wins. I don't want him to win."
Labour MP Michael Wood asked Tipple to justify his stance to the Muslim community and their right to feel safe.
"Nobody will ever be completely safe from a lone madman who is determined to cause murder," Tipple replied.
"This wasn't about killing people. It was about dividing people."
National MP Judith Collins asked how many firearms Gun City had sold like those used by the mosque gunman since March 15. Tipple said it would be dozens, not hundreds.
Tipple said it was "very clear" to those customers that those firearms would likely to made illegal.
He said he had never sold a large-capacity magazine with an A-category firearm.
He said he used to think large-capacity magazines should not be so tightly regulated, but he had now changed his mind.
Unless generous compensation was offered, gun owners may not want to hand over their guns to police, he added.
Tipple, who has said Gun City sold four firearms to the man accused of the Christchurch shootings, said after his appearance that there were legitimate reasons to have an AR-15, the gun that was used in the Christchurch massacre.
"Because why judge a gun on its looks. Why did we write a law that said military-style. I don't like your style. What does style mean? We shouldn't judge a gun. When I showed it to John Key, he said, 'Would they be happy if they painted it pink?'"
He said it was "understandable" that his comments were causing upset in the Muslim community.
"Which one of us, after this event, doesn't have a warm-fuzzy when we see a hijab? Have we all grown in empathy with that community because of that event? That's how we beat this mad Australian, when we get together and not divide, when you stop lynching me as a gun-owner."
Collins told media outside the committee room that Tipple had appeared to have gone to great lengths "to look more reasonable", but was skeptical about how Gun City has conducted its sales since March 15.
"I don't think for a moment that everything we heard was necessarily what has been practised over the years, so I'm going to look forward to what we hear back from the officials about how they see Mr Tipple's evidence."
She said the buyback scheme, which is still being worked out, would probably cost more than intended, much like renovating a house.
She said it was questionable whether Tipple was a fit and proper person to own firearms, given his own trouble with the law in the US.
In 2002, Mr Tipple was arrested at Los Angeles airport after US Customs officials found 29 guns and 340 rounds of live ammunition in his baggage.
He pleaded guilty and was convicted of failing to notify an airline in writing that he had firearms in his luggage.
He was also indicted on federal firearms charges for illegally buying 363 rifles and shotguns from a US gun dealer.
The charges were eventually dropped and Mr Tipple was allowed to return to New Zealand.
Opponents will 'never outweigh the 50 reasons that brought us here today'
Federation of Islamic Associations NZ president Mustafa Farouk spoke before Tipple and told the committee that he was not an expert on weapons or to talk about what weapons the bill would ban.
He was before the committee to represent the people who died in the Christchurch attacks.
The shooter used the kinds of weapons that the bill would ban, he said.
Although people should have hobbies, Farouk said they should consider the greater good rather than the individual good.
He said the bill was only the starting point.
Muslim man Brent Smith, a former army officer who worked in weapons safety, told the committee he fully supported the bill.
Only the military and the police should have access to the weapons the bill will ban, which are designed to shoot powerfully and rapidly.
He also supported the proposed ban on the parts that could convert other firearms into MSSAs and assault rifles. He had concerns about gun storage, too, and collectors had had weapons stolen that were "supposedly secure".
Rehanna Ali, also from FIANZ, said the tragedy of March 15 had provided clarity to act on gun law reform.
The reasons to oppose to bill "will never outweigh the 50 reasons that brought us here today," she said, referring to the 50 victims who died in the Christchurch terror attack.
"We also represent the general NZ community. We want to reclaim the country we had before March 15 ... the outpouring of compassion, of shared grief ... has gone a great way towards reclaiming who we are, and this legislation will continue on that path."
Farouk said New Zealand had to do "whatever it takes" to prevent another March 15. He asked why people needed MSSAs and assault rifles if police in New Zealand could be generally unarmed.
Farouk said the Muslim community had been inspired by New Zealand's response to what happened on March 15, but if the bill failed or was delayed it would upset the community.
Smith said the parts of would-be banned firearms should also be banned, which the bill proposes.
Tipple earlier week this took another shot at the reform, stating that rushing the reform was causing division.
"Rather than divide us, this tragedy has actually created unity and a beautiful empathy within New Zealand.
"I would like that unity and empathy to be extended to legal gun owners, our employees and our families. Rushing this law is causing division," he said in a video statement posted to social media on Monday.
Tipple believes banning the guns is the wrong move, and instead is advocating for increased education, tougher laws and penalties, and tighter gun controls.
"We didn't ban big white trucks after the Nice murders, we didn't ban school trips because one caused a death, we didn't ban holiday flights because one killed hundreds - instead we look at the causes and we take steps to avoid similar tragedies," he said.
"Every society has guns, and there is no correlation between the forceful reduction of gun numbers and an improvement in public safety.
Earlier today the police union told MPs the speed of gun laws changes, if anything, has been too slow.
Parliament has been both lauded and criticised with the haste with which the bill is progressing, but Police Association president Chris Cahill said successive Government had been far too timid.
"Is it time to act? Are we being to quick? No, we've been far too slow."