Shooting incidents in London have almost doubled compared with the same period last year, prompting grave concerns that gun crime in some areas is out of control.

Scotland Yard has admitted a 17% rise in overall firearms offences, although the Metropolitan police chief, Sir Paul Stephenson, has put that down to a blip.

Stephenson last week dropped a plan to put routine armed patrols on the streets, saying that although gun crime had risen it was still lower than two years ago and he was not willing to sanction such a dramatic departure from the principle of unarmed policing. However, the true scale of the increases is contained in figures being presented on Thursday to the Metropolitan police authority.

They reveal that the number of actual shootings has almost doubled from 123 to 236 in the last six months compared with the same period last year, a rise of 91.8%. Serious firearms offences have risen by 47% across the capital.

Those discharging the firearms and those being shot at are young teenagers involved in "respect shootings" to settle petty disputes with little thought of the consequences, say police and community leaders. More and more such shootings involve a gunshot to the leg, echoing the punishment attacks meted out in Northern Ireland. Detectives say that pointing a gun at the shin or knee is a deliberate tactic to avoid any chance the victim will die, risking a 30-year mandatory sentence.

Those working at local level in the capital say the number of incidents reported is only a fraction of what is taking place.

"Gun crime has never gone away," said the Rev Les Isaac, who works as a street pastor in south London. "Firearms are being discharged more or less on a daily basis in some parts of London."

He added: "Those using the guns have got younger. These children are unpredictable, they have access to guns and they are willing to use them."

Instead of relatively expensive guns from eastern Europe, the past weapon of choice, the younger perpetrators are using cheaper shotguns and converted handguns as they are easier to procure and ammunition is more readily available.

The figures contrast with those nationally, where gun crime is still falling in known hotspots. On Merseyside, gun crime is down by 27%; nationally, firearms offences have fallen by 5%.

Pastor Nims Obunge, a member of the independent advisory panel that works with Trident – the Met unit that investigates gang shootings – said the motives of the perpetrators had changed.

"We are not just dealing with drug turf wars here. We are talking about young teenagers who no longer use their fists to settle disputes. They say, 'I can get access to a gun, why would I not use it?'"

Joanne McCartney, Labour spokeswoman on policing on the London Assembly, said: "It is extremely concerning and what I'd like to see is some more information on the age of the young people involved. We need to ask whether there has to be a shift in resources and whether the focus which was on knife crime has now to be moved to gun crime."

Part of the increase in firearms offences in north London was caused by a Turkish gang war which led to three fatal shootings as two groups fought for supremacy. But in other areas the explosion of firearms incidents is harder to explain, or to tackle.

"Where you once had a set of gangs in a particular area which you could define, now we are seeing these broken down into further divisions with several smaller groups forming in particular housing estates all with their own rivalries," said Isaac.

Cressida Dick, assistant commissioner specialist crime, admits the figures are disturbing. "The increase in firearms discharges has continued to be a matter of grave concern," she writes in her report to the MPA committee.