The pub on the corner may be called the Elephant and Castle, but I'm a long way from home. I met Pam Bosley at St Sabina's Roman Catholic church in Chicago's south side. Her college student son Terell was shot dead by a stray bullet back in April 2006. As he was getting his drums out of the van in a busy street, he found himself in the middle of a gun battle. His murder remains unsolved. A photo-montage on the front of the church shows dozens of faces of local people who have been killed in random acts of everyday violence. The photo include one of Jarvis, the foster son of the parish priest of St Sabina's, Fr Michael Pfleger. Jarvis was killed by random gunfire in 1998. One weekend last month, 49 people were shot in the central Chicago area, with 10 dead.

Things are nowhere near this scale in south London, where I am a parish priest, but the same violence exists. In April, three local young men were convicted of shooting a five-year-old girl who was caught up in a turf war between the Brixton-based Guns And Shanks gang and their rivals, the Stockwell All 'Bout Money gang. Much of this gang culture originates in places like Chicago. In London, the police are know as "feds", and gang language is lifted straight out of US gangsta culture. So what does the US have to teach about how to tackle all of this?

"America is crazy for guns. We love guns more than life," Pam Bosley tells me, explaining that since the loss of Terell she has toured local schools giving talks on gun violence. The street price for a handgun round here starts at $25. Just to compare, a packet of cigarettes costs nearly $10. The common scam is to go and buy 100 or so guns from the local store, declare them stolen, claim the insurance and then sell them off cheap. Judging by a show of hands in the schools, the majority of pre-teens in the area know how to get hold of a gun.

The logic that justifies this astonishing availability of weapons is that of security: violence will be discouraged if the violent know their victims may themselves be armed. Guns keep us safe, is the line. The photos outside St Sabina's show what a terrible lie this argument is. But it is a lie so well established, so politically persuasive, that it permeates all the way from the desolate streets of the city's south side to the Nato summit meeting today in McCormick Place just up the road. Fear justifies the need for security. And the need for security justifies more spending on guns. For Nato, it used to be the Russians, but now it's Iran.

The problem with this logic is right under the noses of Nato leaders, should they venture out of their compounds. The murder rate in Chicago is up 54% this year. The Windy City now has more murders than New York which has more than twice the population. This is mostly down to the fact that the place is awash with guns and fear, with each of these continually justifying their own existence with respect to the other.

"What do the politicians say when you lobby them about guns?" I ask Pam Bosley. Most of them don't understand and don't care, she insists. The new mayor of the city, Rahm Emmanuel, thinks the answer is to put more police on the front line. But Pam believes this won't make any difference. There is a code of silence on the streets. Despite the many eye witnesses when Terell was shot, no one offered information about the killers. "We have become desensitised to the violence," she says. Thank God we do not have the same idiotic culture of gun ownership as they do in the US. But desensitisation creeps up on a place. And a code of silence certainly exists among some. The fact that violence is not at Chicago levels is nothing to take for granted.