In that case, you request a Form 4314C, Customer Complaint Form which you complete and submit for the Postmaster to follow up on. Firearms can be sent through USPS, and no, the floor employees cannot open your package just because they are suspicious of its contents. To do that requires getting the Postal Inspection Service involved and as many parcels as we handle, none of us are going to take the time unless some thing is really out of place or suspicious looking. My Postmaster - my boss - recently advised me when I was shipping three rifles out of state that belonged to a close friend to package them well, no markings to indicate contents and insure the [beeep] out of them. Beyond that, I was good to go. None of the clerks at the counter gave them so much as a second look and I wasn't queried as to the boxes' contents. Took them four days to get from KS to AK, without a scratch.
I would like to say, for the record, that I am biased as I work for the US Postal Service. Some of the ways the "company" is managed leave a few things to be desired, but I can assure you that the rank and file employees are genuinely concerned that what you send arrives at its destination in a timely manner and exactly as you last saw it when it was packaged to ship. Those packages I do see damaged in transit are ALWAYS because the sender did not take the time to properly secure the contents. For example, guns should be in a well padded hardshell case, locked, with the owner's name printed on a card inside, should something happen to the package. Also, remove the firing pin assembly or bolt and send it in a separate package in case something should happen (this applies regardless of shipper). Record serial numbers of all guns being shipped and, on the off chance that the gun comes up missing, it is useless without a bolt or firing pin assembly and in order to get one, you have to order it. Bolt guns require the serial number to order a new bolt and often, when those orders are placed, the gun is run NCIC to make sure it isn't stolen. I was told this by an Alaskan guide and, from what I've heard in follow up, it's sage advice.
When you send a gun (or anything) USPS, always insure it for its value (over $200 to get the blue INSURED label), send it Priority Mail with USPS Tracking. I can assure you that with that many bar codes, that package will get handles with tender loving care and with the new USPS Tracking Number it can be tracked from the point of origin to its destination and all points in between. If possible, send it via "REGISTERED MAIL" since that is the most secure form of mail, and far more secure than anything the other carriers offer. I once carried a canvas bag of Registered Mail that contained $110,000 to a bank for delivery. We don't do that now, but when I first started carrying mail, I handled bags of money up to three days a week If Registered Mail is lost, management goes back to its last known place and tears the place apart. Whoever lost it will be fired. Period. The point is - our jobs are on the line when we see Registered Mail and we watch them like they're our only child. Having said that, there are restrictions to how a Registered package can be wrapped and packaged so check with your local Post Office for details.
As far as package security, I am also a taxidermist and UPS is well known by guys in my business for opening packages with oversized forms, cutting them down to fit into multiple smaller packages then shipping them on. Postal employees cannot open packages. It simply does not happen.