With apology to the OP. This applies in your case also
Acknowledging that a gun is an inanimate object, the difference between it and a car is that a gun was designed and created for the purpose of destruction (generally killing).
First, take notes of everything she says in her argument. Then tell her,
A gun is designed for killing, yes...
You can kill game to feed yourself.
You can kill a serial rapist and preserve the dignity of yourself or another, as well as save families from the anguish of a loved one being brutalized.
You can kill an unlawful intruder who has broken the sanctity of your home and threatened your family.
You can Kill tyrants to preserve your freedom and your way of life.
You can kill a raging grizzly bear who WILL eat you if you do not.
Now, get ALL of your kitchen knives out of the drawer and repeat back every argument she has made to you, except replace "guns" with "knives".
Make the same argument that every gun grabber makes. Such as,
Those things make me feel bad...
There are just too many of them...
They are just too easy to get...
Don't you think there should be background checks...
Those things are only designed for stabbing and slashing...
If it saves even one child's life....
Don't expect to "win" and argument with her. Gently persuade her and show her there is another way to think about it.
Then take her to the range!