Are you sure of this ... I'm being told over at the "firing Line" Forums that it's illegal because it violates the 10 imported part rule ... thus even adding a USA Made Mag extension is illegal.
A shotgun with a telescoping stock or a magazine greater than 5 rounds is prohibited from importation into the United States under the provisions of section 925(d)(3) of Title 18, United states code (U.S.C.). Assembly of such a shotgun from imported parts is prohibited under 18, U.S.C. Section 922®. The implementing regulations in Title 27, Code of Federal Regulations, Part 478, Section 478.39 (formerly Part 178, Section 178.39) prohibit assembly of such a shotgun that contains more than 10 of the imported parts listed in paragraph © of the regulation. Please see the enclosed brochure for further information. Assembly of such a shotgun using 10 or less of the listed import parts is not prohibited. The shotgun is also subject to whatever State laws and local ordinances may apply.