USPS will ship a rifle, and might be cheaper than UPS.
That said, go to the UPS site and do a search for 'firearms' the first result returned is:
http://www.ups.com/content/us/en/re...s/firearms.html?srch_pos=1&srch_phr=firearms
I've edited out parts that might not pertain to you...
Shipping Firearms
Use These UPS Services for Your Firearm Shipment
UPS accepts packages containing firearms for transportation from and between ... persons not otherwise prohibited from shipping firearms by federal, state or local law, and when such shipment complies with all federal, state and local laws applicable to the shipper, recipient, and package.
UPS accepts firearm parts for shipment, provided the part is not a "firearm" as defined under federal law. (Note: Receivers ...are considered "firearms" .)
Follow These Packing Requirements
All firearms are to be shipped in new corrugated packaging which meets the UPS Single Wall Box Strength Guidelines. Medium and large UPS boxes may be used for smaller shipments being shipped via UPS Express service.
All firearms are to be packaged in accordance with UPS Packaging Guideline Specifications.
About Documentation and Labeling
The shipper must use Delivery Confirmation Adult Signature Required service for each package containing a firearm, including a handgun, and affix a UPS label requesting an adult signature upon delivery.
The labeling and outer box markings on all firearms shipments should not identify the contents as containing a firearm. Labeling, including the shipper's and consignee's abbreviated names on the shipping label or air shipping document, must be non-descriptive.
Getting Your Firearm Shipment to UPS
Firearms (including handguns) may be shipped only through a UPS Scheduled Pickup Account (specifically, Daily Pickup, Daily On-Route Pickup, UPS Smart Pickup®, and Day-Specific Pickup), or through a UPS Customer Center (counters at UPS operational facilities). Note: Firearms (including handguns) are not accepted for shipment via, UPS Drop Boxes, UPS On-Call PickupSM, or at locations of The UPS Store® or any third party retailer.
See the terms and conditions in the UPS Tariff/Terms and Conditions of Service for information regarding firearm shipments.
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If you decide to ship USPS, I recommend you go to their web site and do a similar search.