As soon as the fighting starts, Gavroche wants in on the action. Walking down a street, he sees a man's pistol lying unattended and snatches it up, waving it in the air and singing revolutionary French songs.
He only realizes later on that the pistol has no hammer mechanism, so it can't fire. (We think this just might be a metaphor for something.)
Before finding the French fighters, he makes sure to visit the barber who was mean to him and his two urchin friends earlier in the book. He picks up a rock and breaks the barber's window, peeling away before the barber can see who did it.
Meanwhile, all the dudes in the ABC Society are marching toward a barricade in a nearby part of Paris. As they go, they run into the old Monsieur Mabeuf, who wants to come with them and join the fighting.
They tell him to go home at first, but he insists on fighting with them. Remember, this is the same dude who wanted nothing to do with anything political earlier in this book.
Courfeyrac comes out of his apartment and finds a mysterious young man waiting for him outside. The young man wants to know where Marius is, but Courfeyrac doesn't know. The young man then asks if he can follow Courfeyrac to the barricade and Courfeyrac figures that it couldn't hurt to have more people on the side of the revolt.