What I would suggest Mark, is the clock needs to react to the impact of the magic bullet, therefore the clock should bounce off the flat rather than just fall.
If you attach a spigot to the back of the clock, as the hanging point, then have an extension spring over the spigot, which places the clock under tension to the flat.
Push the clock against the flat surface, as the spigot passes through the hole in the flat & have some one else push a pin [nail] through a hole drilled into the protruding end of the spigot that?s through the flat. Attach a pull string or cord to the pin, when then holds the clock in place & captivates the spigot.
When the guns report is heard, pull the pin out, from behind the flat & the clock is propelled forward by the spring tension & hits the floor.
I would suggest using a brass threaded bolt, as the spigot, because it is easier to drill the pin hole, than it would be in a steel thread. Use nuts to captivate the clock end of the spring to the spigot. If it is difficult to remove the back of the clock, in order to mount the spigot. Make and attach a false back to the clock.
The pull string/cord can be attached to the nail [pin] by bending the head end so it can be tied on.
Also just rub on candle wax on the nail & ream or de-bur the drilled hole, to reduce fouling when the pins pulled.
I hope you can understand my explanation?

Dont know the show... Is it a drama or a comedy.. I don't have a "real look" fix up my sleave, but for comedic effect we did a gun shot hits a picture in a comedy where the picture was mounted on a pivot that was connected through the back of the flat to a battery powered drill... when the shot rang out, it spun.  Of course that kind of thing is only good for comedies.....

 

 

A good, old-fashioned rat-trap (similar to a
mouse-trap, but larger) nailed to the shelf beneath
the clock.   

Set the spring and attach a concealed wire to the
release lever. On cue (or "gunshot") yank the wire
from offstage to spring the trap. Depending on how
heavy your clock is, the action of the trap will be
violent enough to rock your clock.

If you have the time or budget, the same effect can
be used to shatter the clock's crystal, knock out its
face, or cause any number of damage to
knick-knacks or whatnots on your shelf.