The taser "stun gun" which was used for the first time on Sunday by police officers dealing with an armed man did not work.
Scotland Yard said officers went to an address in Hounslow, west London, at about 1045 BST after a call from a man who said he had a gunshot wound.
When they arrived they found the man carrying two handguns.
They tried to negotiate with him for about 45 minutes and then decided to use a plastic baton round to disable him.
He was hit in the stomach and fell to the ground but when he tried to get up again a taser was used.
Shooting under investigation
But Inspector Glen Smyth, chairman of the Metropolitan Police Federation, said: "It didn't work in the way that it should have.
"You have to have both parts next to the skin or the clothing and if they aren't it won't work
"What we don't know is whether it didn't work properly because of the way it was aimed or if there was something defective about the second barb.
"It doesn't mean the weapon doesn't work; it just means in the particular circumstances it didn't work."
The shooting will be investigated under the supervision of the Police Complaints Authority.
The man involved in the incident was arrested and is now recovering in hospital.
Police say knives, firearms and ammunition were recovered from the address.
The taser, which transmits an electrical impulse into the subject's body, is designed to be a "less lethal" way of disabling a suspect.
It fires out two darts up to 21 feet and transmits an electrical impulse into the central nervous system of the subject.