Essentially, double-effect means that your good actions can have unintended yet foreseen evil consequences. If you make a hunting rifle, and someone uses that to shoot children at a school, are you responsible? Not an easy question to answer.
Someone driving a car into a crowd does not make it Ford's fault. Society does not ban cars just because 50,000 people a year are killed by them.
Double-effect has four criteria for making a judgment in any particular case. The intent of the maker is only one of the four. The most pertinent criteria in the case of the hunting rifle and school shooting scenario is the last, of proportionality. Does the good to be gained by having hunting rifles exceed the evil to be suffered by school shootings?
That's a tougher question to answer than whether there is an ill intent.
Perhaps it's the European in me that can't wrap my head around the American idea of guns. To me "Guns are dangerous" seems obvious.
It's an interesting cultural difference.
Personally I think that although this idea makes sense for knives, guns cannot be justified because they have no purpose other than killing (hunting is still killing).
Suppose someone is trying to kill you or your loved ones. Do you really not see how a gun could be beneficial? Do you really not see how killing someone could be the right thing to do?
On the front page is a story about someone recovering a camera from a thief. One comment says that confronting the thief could be dangerous, since they might have a gun ( https://news.ycombinator.com/item?id=5505464 ). Here in Ireland if some low level criminal stole my camera, they wouldn't have a gun when I meeting them in a café.