The headline seems okay to me. Gunfire, the firing of guns (rather than the sound of the guns), has been deliberately stopped. A similar use of "silenced" might be applied to dissent or dissentors. The Czar's critics were silenced. To say that the guns were silent is to suggest a passive or happenstance result. The fact that there exists a device known as a silencer for firearms does muddy the waters slightly, but not so much that a reader would interpret the headline incorrectly.