The point is that the rate of loss is low enough that whether you engage in risky or safe practices will have no effect on your premium. Thus you're asking legal gun owners to pay the huge bill of gun crime.
Can I collect from you for the uninsured bastard that hit me? (Admittedly the statue of limitations is long expired.)
The uninsured bastard that shoots you didn't get his gun from a legal source: if he did, the fact it was no longer insured would mean that it would have been taken away.
The person who DID get that gun from a legal source would pay higher premiums on guns they claim to continue to own. Perhaps they would no longer be able to but guns at all depending on the circumstances. The seller's who sold those guns, especially when they sell many guns that end up falling out of visibility, would then ALSO pay higher premiums.
You are claiming the "loss rate" is low. It is not: it is high enough to account for ALL the illegal guns on the street, which accounts for MOST of gun crime.
Further, this assumes that the insurance company does not sue and recover from the straw seller's for breach of policy: that the owner is not still held liable; insurance doesn't cover criminal acts of the insured, including to give a gun to someone who does not hold proof of insurance or failure to disclose the transfer; if they fail to disclose, they get their gun taken away, and if they don't have it, they just committed a crime, have ruined their ability to pass a background check, and opened themselves up to personal liability.
Nobody wants that, so as a result, few people will continue allowing their guns to enter the black market, and the people who do own guns legally will be sure to properly secure them.
This is predicated on the notion that most illegal guns come from straw buyers and the like, not theft.
