I used to drive truck and carried a lot of USPS loads, so IveI been inside a lot of their terminals. They are like every other shipping company, they have no care or regard for the packages.
I hate to say this but good luck with your claim. Also since it was ammo, I hope you had it marked correctly, if you didn't I would not say another word and just take the loss. A fellow I worked with nearly did jail time for sending ammo through the mail.
In fact I don't think it's legal at all to ship ammo via USPS
Sorry just looked closer and see it's brass and bullets.. I'm on my phone, so picture is small.