If you removed the cylinder, it would be considered a part and could be shipped like any other package.
To be legal shipping a complete gun, there must be a FFL with either the shipper (you) or consignee (Trijicon). If it is a handgun, it must be shipped priority (USPS), or at least 2nd Day (UPS/Fed Ex). If you are shipping to Trijicon, I would ask for a copy of their FFL to avoid any problems with you shipping the gun. Chances are an adult sig will be required on return. If you removed the cylinder, it would be considered a part and could be shipped like any other package.