Because gun owners love conspiracies, there's a widely-held one in Fuddlore that the "sporting purposes" clause was thrown in as a sop to the gun manufacturers of the Northeast, who were having a hard time selling new Remchester deer rifles at high retail prices in a market where it was easy to buy and Bubba-ize a Mauser. Regardless of the factuality of this origin myth, the outcome was that formerly cheap guns became expensive collectibles in relatively short order, making a domestic hunting rifle seem a more economically viable purchase.
In the Eighties, the import floodgates were opened again because an exemption had been added to the "sporting purposes clause" for firearms that were designated as Curios & Relics, either directly by name or via age.
Someone once told me that this had been part of the Firearm Owners Protection Act of 1986 and I'd just taken that assertion at face value. I'd never been able to find the language in FOPA '86 myself, but I'd always assumed that was because I wasn't a lawyer and it was hidden in the emanations of some penumbra or another.
The other day I was reading through David Hardy's classic scholarly piece on FOPA '86, and came across a footnote referencing "the Act of Oct. 30, 1984, Pub. L. No. 98-573, 98 Stat. 2991-92, which allowed importation of most military surplus arms that qualified as curios and relics."
Hmmm...
Plugging "Act of Oct. 30, 1984, Pub. L. No. 98-573, 98 Stat. 2991-92" into Google took me to a typical dull piece of legislation (big PDF) that fiddled with and adjusted trade regulations and tariffs on everything from "fresh asparagus" and "steel pipes and tubes used in lampposts" to "unwrought lead" and "hogs and pork products from Canada".
There, buried way down in section 233 was an adjustment for "certain curios and relics". Scrolling laboriously through the text took me to section 233, which stated the following:
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