Welcome to the site.
I suggest you contact your local or state Rifle/Pistol Association, who may have links to, or be willing to provide a referral to, an attorney experienced in matters related to firearms and 2A. Most attorneys will give you a free consultation, hear the basic facts of your situation, provide you an estimate of success, and an estimate of the time and cost involved in pursuing your case. You may be VERY surprised at how much the situation improves once you get an attorney involved.
Lesson here: every single time you buy a firearm, whether or not you schedule it with your insurance company,always keep a copy of your paperwork complete with serial number and bill of sale. Doesn't help you here/now, but for the future. Your gun safe is an excellent place to store your accumulated records. Several companies make excellent products that hang on the door, conserving space and usually also provide document pockets and a pistol rack.
This is a shame. Good luck with it.
"To my mind it is wholly irresponsible to go into the world incapable of preventing violence, injury, crime, and death. How feeble is the mindset to accept defenselessness... How pathetic." - - Ted Nugent
"Cogito, Ergo Armitum Sum" - (I Think, Therefore I Am Armed)- - anon.