Long version
https://docs.google.com/document/pub?id=17ixogftgITEblNUWtmFBv96ZvgjK6eFell8GsAWd-KI

Short version
The McKinley Tariff of 1890 set the average ad valorem tariff rate for imports into the United States at 48.4%. Sporting, breech-loading double-barrel shotguns had a 35% ad valorem PLUS an import duty of $1.50 if priced less than $6; $4 if $6-$12; and $6 if priced greater than $12. Single-barrel breech-loading shot-guns had the same 35% ad valorem PLUS an import duty of $1.
Forged rough shotgun barrels i.e. non-joined tubes, however, were exempt from the tariff which allowed the US makers to continue to import Damascus tubes from (mostly) Belgium to fit and finish here.

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