Considering I have yet to hear or read of an agency that will offer such insurance, I wonder how you've come to this conclusion.
But doesn't it trouble you that the free market insurance industry apparently doesn't think gun owners are a good risk?
If the problem is small, then the risk is low, then the premiums should be affordable.
If the problem is great and the insurance industry doesn't want to touch it, well, that doesn't speak well of gun ownership in the US.