Stop emailing and phoning and use registered mail, which gives you PROOF of delivery/acceptance of your demand to have your stolen property returned.
They can ignore phone calls and emails till the cows come home because obtaining proof of those will be just as hard to obtain as the gun. But the USPS will provide you with a handy dandy return receipt/proof of delivery (with signature!) that is in your possession rather than theirs.
Seems stupid, but snail mail creates a paper trail they can’t control or expunge. Which means they are more likely to pay attention and deal with it appropriately.
Sensible advice. Thanks.