I already pointed out the impossibility of that. Guns can be in operating condition for well over 100 years, and you can't compel someone to maintain insurance on something they no longer possess.
I believe that was dealt with upstream.
All you need is something similar to a SORN with regard to cars, i.e if you're no longer the owner you notify the registration authority and confirm that either the gun has been destroyed or has been sold/given to someone else. You can then cancel the insurance.
In a perfect world the next owner of the weapon would then have to register it and insure themselves.