Yes, the brass tube is what would be considered the receiver, that is where commercially made gatlings have the required manufacturer/caliber/serial number engraved. Manufacturers are required to stamp/engrave that information on any firearm they make and keep detailed records.
A homemade receiver would present a real problem for a FFL to ship, there is no Manufacturer/model/serial number or caliber markings to enter on the required records/forms a FFL is required to keep.
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Further, going by the letter of the law, a FFL shipping it is actually called a \"transfer\". Laws allow one to build a homemade gun for their own use but transferring it moves one into the territory of being considered a manufacturer. A manufacturer needs all sort of licenses and conditions to be legal not to mention extensive record keeping and much more \"red tape\".
I will assume the questions are for academic discussion purposes only. In reality, the practical thing would be to ship a homemade gatling receiver using the description of \"machined brass pipe\" and avoid creating a lot of unnecessary problems.
