As a note of credibility I'll start by telling you that I spent 20 years as an insurance claims adjuster, followed by 30 years as a property and casualty insurance agent. Now, some facts. Any liability insurance policy follows the basic rule of insurance requiring the occurrence be of a "fortuitous" nature. For those of you proposing this ridiculous gun insurance, that means the act is accidental in its being. Furthermore, it is a fact all liability policies specifically exclude coverage for any intentional act of the insured that causes damage or injury to a third party. One can't insure the gun for its action it's just a chunk of iron.
Assuming the discharge of a firearm causing loss, damage or personal injury was in fact accidental, coverage is predicated on negligence (liability), not the mere ownership of the firearm.
Specific gun insurance makes no sense. A homeowner's policy or renter's insurance already has a personal liability section built in subject to the above provisions.