Early Firearms: If an early firearm with the broken condition misfires again, it explodes. When a nonmagical firearm explodes, the weapon is destroyed. Magical firearms are wrecked, which means they can’t fire until they are fully restored (which requires either the make whole spell or the Gunsmithing feat). When a gun explodes, pick one corner of your square—the explosion creates a burst from that point of origin. Each firearm has a burst size noted in parentheses after its misfire value. Any creature within this burst (including the firearm’s wielder) takes damage as if it had been hit by the weapon—a DC 12 Reflex save halves this damage.
So, if you fire a broken firearm (whether broken by misfire or anything else), it explodes. The number in parenthesis is how big the blast is.