In 1857 the design was adopted by the U.S. Army as the light 12-pounder M1857. The Union made 1,156 units while the Confederacy made 501. Because they did not have the manufacturing capacity of the North, Confederates attempted to capture as many Union-made Napoleon 12-pounders as they could. The Confederate army also developed their own version. In 1863, General Robert E. Lee was so impressed by the M1857 that he had all six-pounders in the Army of Northern Virginia gathered up and sent to Richmond to be melted down and re-cast into 12-pounders. The Confederates kept producing 12-pounder Napoleons until the Union army captured the Ducktown copper mines near Chattanooga, Tennessee. This reduced Confederates bronze production. After that Confederate 1857s were cast of iron.