REVOLVER, by Gordon Lawrie

Amelia still isn't sure what it was she saw that day exactly, but it looked like and felt like a gun: something very similar to the Smith & Wesson 625 her unfaithful husband had once possessed. 

There it lay, reflected in the dressing-table mirror, a six-cylinder revolver with a dark handle, and although it seemed fully loaded, she thought it had  the safety catch on.

Concluding it was probably a replica anyway as she studied it, she pointed it randomly in different directions. In fact, she only decided it was real when it blew her brains out. That settled things.