Not out one penny, you will actually get paid for the amount of the machined parts you insured them for. If they find them then your package is delivered and you don't need to file a claim.
Rifle parts and firearm parts are just "PARTS". What is considered a firearm is the ACTION with a SS#.
When it comes to an insurance claim from a lost UPS package even though you insured it for XX $s as machine parts, UPS will want to know what the parts were to justify the value. So if sending a rifle, and it gets lost, you will need to come up with some machine parts. Could be a trigger, barrel, stock bottom plate, ect. You will get paid if filing a claim. The part that will get you in trouble with the law is the lost rifle - receiver with the SS#. If it turns up and the ATF tracks it back to your package you will be in more trouble than you ever expected. Federal Charges and Felonies. Good by to any current/future employment. Better have a good lawyer and bank account.