I like it and think it's warranted to handle many irresponsible gun owners.
The more firepower & number of guns - the more liability insurance required. After all, if someone owns 10 guns, they have more liability than someone with 1 ! Can you buy a single car insurance if you own 5 cars? Might be 1 policy but the cost is much higher to cover all vehicles.

AND insurance doesn't STOP your right to bear arms - just places responsibility on YOUR actions.

Great idea. Also might prevent stockpiling !
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David (OFI)