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.

Nov. 30, 1895 Sporting Life
http://www.la84foundation.org/SportsLibrary/SportingLife/1895/VOL_26_NO_10/SL2610011.pdf
"How Shot Guns Are Made and the Process Through Which They Pass Fully Explained"
The beginning of the manufacture of a gun is the barrels, and it is generally known that no barrels are made in this country except the rolled steel, which is used on the Winchester gun. All gun barrels are now imported, although an attempt was made a few years ago to produce them in this country, but with only partial success. England, Germany and Belgium supply most of the barrels, the latter country doubtless producing the larger quantity.
All gun barrels, whether imported direct from the makers in Belgium, or through an importer in this country to the gun manufacturer, are received in rough tubes, which very much resemble a couple of gas pipes, but being somewhat larger at one end than at the other. These barrels or "tubes" as they are called, are merely tied together in pairs, with small wire and 40 to 50 pairs are packed in a box.

Infro re: Damascus barrels and ribs here
https://docs.google.com/document/d/1zEE1xuaNuR26Drg3E_IMUEpA_YThLMP0VSIx2xJM9bA/edit

https://sites.google.com/a/damascusknowledge.com/www/home