Tell em the truth, in detail..
IF you aren't allowed to mark the exterior of the package "GUN" what's the point of telling the agent? Do they mark the package or the paperwork al la airlines that put a big "LOOKY HERE" sitcker on guns declared for transport. If asked, tell them the truth,...it"s a MarkIII Mod 4 evil reducer or similar verifiable nomenclature like verbage. As for insurance, your household insurance covers your personal property "anywhere in the world". Read your policy instead of using it to line the drawer bottom.