Stormdog is absolutely right: This has less to do with firemarms and the Second Amendment than it does with premesis liability. Under the common law dating back to Colonial England a landowner is liable for injuries sustained as a result a known dangerous condition especially if the injured person is an invited guest and the injuries are the foreseeable consequence of the dangerous condition.
Also is the concept of “reasonable person.”
Is reasonable to assume that a person would pick up a rifle/pistol and then play with it and in the process, point it at another person and pull the trigger?
No reasonable man would do that. Therefore, the fault lies with the shooter-—he was acting stupidly. He was acting unreasonably.