What about suppressors: If I made a suppressor in Florida and kept it always in Florida, would it be legal?
NO (unless you obtained a tax stamp)
Title 26 U.S.C. Section 5845 defines one of the devices subject to the NFA tax as a silencer.
And
Title 26 U.S.C. Section 5821 levies a "making tax" under that title .
Unlike most of the 1968 Gun Control Act that requires firearms to have affected interstate commerce in order to fall under the commerce clause of the Constitution; the National Firearms Act of 1934 is a taxation code. It requires a tax to be paid to
manufacture, import or transfer certain items, regardless of where they are
made, imported or transferred. It is, at it root, a federal excise tax. The federal government requires that tax to be paid and heavily restricts the process by which the tax may be paid (Application, background check, finger prints, fee, etc.)
Article I, Section 8 grants Congress the authority to levy excise taxes (which was the authority Congress relied upon for the 1934 NFA)
AND
Article I Section 8 also grants Congress the authority to regulate interstate commerce (which was the authority Congress relied for the 1968 GCA)
Both acts are gun control acts but they function in different ways.