As EmptyMag said, under ATF's current interpretation of the sporting purposes test in the 1968 GCA, modifying an SL8 to take high-cap mags makes the SL8 an unimportable rifle. Thus, assembling that rifle yourself using imported parts is illegal. You have to incorporate a sufficient number of non-imported parts so that under s. 178.39 the rifle is not considered "imported." For my own SL8 magwell project I am incorporating U.S. made magazines (3 parts), a U.S. gas piston, a U.S. sear, and a U.S. trigger. There is no (separate) barrel extension, muzzle attachment, trunion, pistol grip, or buttstock -- so that is a total of 9 imported parts that are used. Unfortunately the only way to get a definite answer as to whether your project complies with ATF's current (unpublished) interpretation of the 1968 GCA is to write to ATF's Firearms Tech division in Martinsburg, WV. It is my opinion that everyone should do this all the time. That way there will be some pressure on ATF to publish its interpretations. I start every letter with something to the effect of "As you know, ATF has not published its interpretations of these provisions in the Federral Register."
