Dealing with one person who insists the owner should be held responsible if the weapon is used in the commision of a crime after being stolen.
Tell them it's the same as if their car/baseball bat/kitchen knife was stolen and used in a criminal action. An object is an object. Plus the key
word is stolen meaning you personally do not possess it any longer so ultimately cannot be held responsible. I believe they are confusing this idea with someone leaving a firearm accessible in a residence were children live.
Objects are incapable of operating on their own. There is no question that firearms can help someone commit homicide but so can an ice pick. It needs human intervention though. If they still disagree, write them off as an extreme liberal and inform them that several European countries are accepting applications for citizenship. They'll probably be much happier as there is more governmental control and they have to think less.