The Independent Police Complaints Commission (IPCC) analysed footage from the non-fatal shooting of Nathaniel Brophy, 34, in Clapham on 21 August.
It said the positioning of the cameras on officers' bodies obscured and impacted the quality of the footage.
The Met said it was testing different locations as part of an ongoing trial.
Officers across 10 London boroughs are currently involved in the trial using about 1,000 devices, to boost transparency and speed up convictions, with aims for a further 20,000 to be used by March next year.
Often placed just below the shoulder the camera can be obscured if the officer raises their arm to aim a weapon.
Jennifer Izekor from the IPCC said: "They will be good tools but they're not fit for purpose for firearms incidents at the moment."
The introduction of body cameras followed criticism of the Met over the death of Mark Duggan, who was shot by armed officers in August 2011, sparking riots across England.
The cameras enable officers to store material from each incident for a month which can then be used for evidential purposes.
But Andrew Dismore from the Labour London Assembly group said: "I'm surprised that it's taken the IPCC to point out this particular problem to the Met.
"I would have thought it was pretty obvious that if a firearms officer points a rifle at something that the camera ought to be able to follow what the officer is doing. At the moment they can't do."
The Met said: "The cameras issued to armed MPS officers are capable of a variety of mounting options and these are being tested as part of this pilot...helping to inform any future roll-out.
"No decisions have yet been taken as the pilot is still ongoing and there are still lessons to be learned."
Body cameras worn by armed Met Police officers are "unfit-for-purpose" and should be replaced, the police watchdog has said.