A Labour Party bill was defeated on a rare 60-60 tied vote in parliament on Wednesday night.
The government opposed Phil Twyford's Depleted Uranium (Prohibition) Bill but didn't have the numbers for its usual majority because one of its support parties backed it.
The member's bill sought to ban the possession, use, sale, manufacture and testing of uranium in any weapons or armour.
Depleted uranium, a very heavy metal, is used for munitions manufacture and Labour MPs said it was radioactive and toxic in very high temperatures.
Government MPs said the bill was nonsense.
"This is nothing but a flight of fancy to get that guy in the news," said National's Tau Henare.
"All the scientific evidence says there's no proven link between depleted uranium exposure and an increase in cancers."
The bill failed on its first reading because an affirmative vote is needed for any legislation to pass.
All the opposition parties backed it and so did United Future leader Peter Dunne, who usually votes with the government.
Mr Dunne on Tuesday voted in favour of asset sales legislation, giving the government a 61-60 majority, and has been attacked by Labour for doing so.