Firearms (along with sewing machines) were one of the earliest products to be mass produced with interchangeable parts. There wasn't a modern assembly line yet, but firearms weren't hand-made in the sense they were during the 1700s, where one gunsmith would forge every part down to the screws. That is probably one of the major causes of the declining prices noted in the article.

https://en.wikipedia.org/wiki/Interchangeable_parts#Late_19t...