Police took an arsenal of unloaded weapons to Parliament last night to show MPs how easy it was to circumvent laws and convert guns into military-style semi-automatic (MSSA) weapons.

The Government moved quickly yesterday to progress the Arms (Prohibited Firearms, Magazines, and Parts) Amendment Bill, which passed its first reading in Parliament with the support of all parties except for Act.

Act leader David Seymour objected to rushing the bill into law and had wanted to make a point by forcing the Government to use urgency to expedite the bill's progress.

But he embarrassingly missed his chance when he was late to the House after speaking to media, later brushing off his error as "something you've got to laugh off".

About an hour after the first reading passed, the finance and expenditure select committee held a public hearing in which police displayed an AR-15, the type used in the Christchurch terrorist attack, as well as a variety of AK-47s and a few shotguns.

Deputy Commissioner Michael Clement told the committee it was simple for anyone to buy unregulated gun components - such as freestanding grip or large-capacity magazine - to turn firearms into MSSAs.

The bill intends to make all these parts illegal, but the Council for Licenced Firearms Owners feared that the bill could unintentionally criminalise 250,000 firearms licence holders.

Spokeswoman Nicole McKee said the bill banned the parts of a prohibited firearm, but it was unclear if all stocks, magazines, butts, silencers and sights that were part of an MSSA - but could be used on a legal firearm - would become illegal.

"Every firearm has a stock. Every firearm has a sight," McKee said.

She conceded those issues could be ironed out at the select committee stage, but it was only hearing submissions for 48 hours.

"There is a moratorium in place stopping the sale and transfer of these firearms. Why is it being pushed through so fast?"

She pushed for sporting exemptions in the bill, which would allow New Zealand to keep hosting international shooting competitions involving firearms that will be banned.

The council was also waiting to hear details of the Government buyback scheme.

"Because the bill is so wide, it's going to affect businesses, people are going to lose jobs," McKee said.

"When we look at adding all of this into [the buyback scheme], it's going to be somewhere between $500 million to $1 billion - and we haven't even started looking at a national register yet."

Police Minister Stuart Nash conceded that the cost of the scheme could be higher than Treasury's estimate of up to $200 million. There were about 14,000 MSSAs in New Zealand, but the number of AR-15s was unknown.

But during the bill's first reading, MPs across the political spectrum agreed that it should be done, regardless of how much the buyback ended up costing.

The bill will ban MSSAs and assault rifles, and shotguns with detachable magazines or internal magazines that hold more than five rounds.

Magazines holding more than five cartridges for a shotgun and more than 10 cartridges for a .22 calibre rimfire weapon will also be banned, as will any other magazine capable of holding more than 10 cartridges.

The Game Animal Council supported the bill, saying it will have "little significant impact" on hunting for tahr, deer, chamois and feral pigs.

National MP Judith Collins supported the bill and called on the Government to go further in its second round of changes to gun legislation, scheduled for later this year.

She wanted police to be empowered with firearms prohibition orders to go after gangs' weapons.

"The best way forward is give police the powers, give them the firepower to it, and get on and take them."

Collins also called for a national firearms register, which the Government has said it will consider along with prohibition orders.

"If I have to register my dog, why would I not have to register a gun? That seems to me totally crazy."

The Government wants the bill to be in force by April 12.