A straw man argument is when you create an argument for the person you are arguing with / against and you set it up yourself in order to knock it down. So you are telling somebody what their position is or you are rewarding what their position is or you're basically just making up their position.

In the case of this argument or this topic it goes like this.

I own a gun for self-defense.

You want a gun for self- defense against people who have guns. If people didn't have guns you wouldn't need a gun for self-defense.

That is the strawman up there when they include or specifically designate the idea that I want a gun for self-defense up against people with guns and that was the pure motivation when that was specifically not stated earlier.