The article was dated Feb. 26, 1916. We are led to believe that the barrel tubes came from Belgium. I have my doubts. I believe they were 'most likely' American made.
The German invasion occurred on Aug. 4, 1914, 30 months before the article was written. Fiscal chaos reined in Belgium during the occupation. Starvation was a major issue. As many as 650K people were unemployed between 1915 and 1918. The Germans looted industrial machinery. Factories were either sent to Germany intact or melted down.
Also, there was talk of a European war as early as 1911. So, if you were the Hunter Arms Company, a rationale business man would try to find an alternate source of gun barrel tubes.
That's what the Hunters did. Will continue later. Might get timed out.