"If people want to have guns, that's their thing," he says, walking to his Center City rowhouse on Sunday.

What he is against, he says, are irresponsible gun-owners, people who use guns to kill or injure others, and "straw purchasers" - people who buy guns legally for the benefit of criminals.

Nacheman, an avid biker, has just finished a three-hour ride to the Wissahickon Trail and Belmont Plateau on his white Motobecane cyclocross bike.

He can relate to people who collect guns, he says, because he collects bikes. He has six in his living room: a mountain bike, a road bike, the cyclocross, his "commuting bike," a tandem ("the only way to get my girlfriend to go with me") and a bike "on display" that he doesn't ride.

Nacheman, who grew up in Basking Ridge, N.J., and graduated from the University of Pennsylvania in 2008, took the helm of CeaseFirePA in January 2011 after "campaigning" (for Hillary Clinton, for an unsuccessful Bucks County candidate and for Mayors Against Illegal Guns).

CeaseFirePA's website calls it "a statewide coalition of survivors and citizens who are dedicated to taking a stand against gun violence."

Nacheman fights for laws that would require, for instance, the timely reporting of lost or stolen firearms. He organizes community members to attend a defendant's sentencing hearing, and calls mayors, faith leaders or town-watch members on gun-related bills.

When he's not doing all that, he bikes, runs and camps.

The 6-foot-3 outdoorsy-type is preparing for the Philadelphia Marathon in November (it'll be his second).

This summer, he biked solo across the state for five days, camping in state forests or parks. He said he passed through cities touched by gun violence, including Reading, York and Lancaster, and through Everett, a Bedford County town where a 15-year-old boy allegedly killed his 15-year-old friend with a gun during a game of Russian roulette in July.

"Everybody's calling that shooting a freak accident," Nacheman says. "It wasn't a freak accident. It was avoidable."

- Julie Shaw