The most used weapon by Union infantry during the US Civil War was the Springfield Model 1861 rifle-musket. This weapon was a .58 caliber, single-shot, muzzle-loading rifle that was widely used by both the Union and Confederate armies during the war. It was a reliable and accurate weapon that was effective at long range, and it was also relatively easy to use and maintain. The Model 1861 was the most widely produced firearm of the war, with over 1.5 million units manufactured.