Complex sentence: It was adopted by the United States and used during the American Civil War, with over 20,000 made, marking the first adoption of a removable-magazine-fed infantry-and-cavalry rifle by any country.
Split into simpler sentences: It was adopted by the United States and used during the American Civil War, with over 20,000 made. This marked the first adoption of a removable-magazine-fed infantry-and-cavalry rifle by any country.