Why is that, though? Think about it, shooting an illusion is no different than waving your hand and creating it with magic. You're thinking on a mundane gun, that barks and creates a lot of noise when shot, but it's a magical gun we're talking about. Whenever Alissa would use it to create an illusion, it would be silent and discreet. Let's try it out in an example.
Alissa is running away from a couple of White Court Vampires who want revenge for Mikhel's death. She is unable to face them directly, since it would be four versus one, so she decides on hiding and escaping their notice, so that she can regroup with her friends and face them on more favourable odds. She skids to a stop after turning a few corners, knowing that the vamps are going to catch up at any moment. Kneeling on the snowy ground, she takes out her trusted pistol, now a symbol of the waning power of the Spring Court. Closing her eyes and murmuring nonsensical words to focus her mind, she points the gun to the ground and shoots; instead of a bullet, a small sphere of light hits the ground, quickly becoming a large dome encompassing her. While she sees the dome, she knows that anyone who watches it from the outside will see only the bare ground and the stretch of wall behind her. She is safe, for the moment.