I also believe most people simply fail to package a gun well enough. Multiple layers are the key. If I ever do mail a gun it will be in a length of 8" Schedule PVC pipe with additional protection inside, minimum.
I have found that those fairly inexpensive plastic gun cases that I buy at Academy Sports work really well for shipping guns, with the guns securely wrapped in bubble wrap inside and soundly duct-taped outside around the cases. Now the cases have gotten cracked and dinged alot, but no damage to the guns at all. I have 3-4 that I keep that I can use for shipping like this. Works like a champ.
I have used this ever since a good friend of mine, and regular contributor here had a pristine, 99% Ithaca NID 20ga, beautiful gun severely damaged by the Post Office, registered mail shipping. Even though it was insured, he was not able to get the value back from the insurance or get the post office to pay the amount of damage. It was not a good situation.
Anyway, since then I am of the opinion that you cannot overkill packing your guns well enough. Greg's PVC pipe idea sounds like a good one to me under that theory.
Brad