Every single firing pin in the NZ Defence Force's 9040 new infantry rifles has been replaced after a number of the rifles broke when the weapon was brought into service.

The firing pin is a critical part of any firearm - without one it won't work.

The rifles were part of a $59 million deal to provide military personnel with state-of-the-art weapons.

The MARS-L rifle was brought to replace the Steyr rifle, which had been the general infantry weapon since 1988.

The new weapon is based on the M-16/AR-15 type weapon and was intended to double the range effectiveness of those firing it out to 600 metres.

The first shipment of the 9040 rifles bought from United States manufacturer Lewis Machine and Tools arrived in May last year.

They have been gradually rolled out since, being introduced as military personnel trained with the new weapon during which time issues with the firing pin emerged.

Then other issues cropped up with about 130 rifles showing cracks around the mechanism which houses the bolt allowing the weapon to be fire.

NZDF confirmed: "Identified issues were the firing pins, trigger mechanisms and cracked bolt carriers.

"During introduction into service training a number of firing pins suffered breakages and to ensure all rifles are now at the highest standard, LMT has replaced all firing pins under warranty."

The replaced firing pins were now at a standard "higher than that of the US Military Standard". As it was done under warranty, there was no cost to the taxpayer.

The $59m deal with Lewis Machine and Tools included the rifles, sights, support weapons and training equipment.

The NZDF order was the largest ever to be filled by Lewis Machine and Tools and came after it bought specialist marksman rifles from the weapons manufacturer.

The firing pin breakages appeared linked to variations in strength across the entire shipment of rifles.

The NZDF statement said: "Root cause testing found some variations in the hardness of the metal firing pins and a new inspection procedure was developed and tested which gave the NZDF a higher assurance that the pins were heat treated evenly."

The firing pin issue, and a number of other issues identified as the rifle was introduced, have not cooled enthusiasm for the weapon.

Commanders and soldiers have offered rave reviews in a recent copy of Army News.

These views were endorsed by Defence minister Ron Mark, who told the Herald the Mars-L was a "vast improvement" on the Steyr.

"I've taken a close look at the Mars-L and also spoken to soldiers who are impressed with the new weapon, which to me is the best sign the project team got it right.

"The previous Government made the right call buying it, so credit where credit is due.

He said "minor issues" with its introduction were covered by warranty and the manufacturer had been "incredibly responsive and proactive in fixing the issues".

Private sector gun expert, Gun City owner David Tipple, said firing pin issues were not uncommon but a variation in the pin across the batch due to manufacturer was "not good".

He praised the rifle and the willingness of NZDF to buy it for their troops when other militaries had settled for the same style but lower quality.

"That rifle New Zealand Army bought was the Rolls Royce. I am so pleased that we made such an intelligent decision."

He said Lewis Machine and Tools was known as a quality manufacturer.

"They're a good company and its a beautiful rifle."

NZDF recently devoted two pages of Army News to highlight the new rifle.