Changing the effect of a certain object but not anything else
You could create one with parts from a hobby store or craft store, and paint them the correct color of gray.  Model your gun replica off of a specific existing gun that you find photos of online. Figure out ways to shoot the scene so the viewers don't see the gun too much:


keep it in shadow
only have it on screen for 1/2 second at a time
keep it moving, especially if this is an action or chase sequence
show it from a distance
keep it out of focus (while something "more important" is in focus, like a person saying a line of dialog)
train the camera on the face of the person with the gun and the reactions of the potential victim


Also use visual effects (as suggested by bret7600 in his answer) and sound effects. A Foley artist once said he uses a door deadbolt to simulate the sound of a cocking handgun.