I use Allstate, after talking to my agent he told me that the only way I could be sure to get full value of the firearms back was with an appraisal. Best to talk to someone who knows what they are talking about with your insurance agency
I would go this route. I have USAA. While I don't have a rider for firearms, I have additional riders for art and jewelry. I had to submit pictures, appraisals, receipts, etc documenting the value of the items.
I would definitely talk to someone within your insurance agency. I would rather err on the side of being over-insured than under-insured.