Guns =/= cars.

>[T]he owner's duty, as in other similar situations, to provide against resulting injuries is a function of three variables: (1) The probability that she will break away; (2) the gravity of the resulting injury, if she does; (3) the burden of adequate precautions.

https://en.m.wikipedia.org/wiki/Calculus_of_negligence

This is just a useful construct for understanding reasonableness.

The issue with guns is that second factor, and to a lesser extent the third.

If someone steals your car, they're likely to go for a joyride or sell it. If someone steals your guns, they're likely to end someone's life with it.