Here he improved the buildings and grounds, increased production while reducing costs, and was instrumental in developing the 1855 model .58 caliber rifled musket which was later to be the principal weapon for Union infantrymen during the Civil War.

[1] With the outbreak of the Civil War in early 1861, Ripley was promoted to colonel (April) and brigadier general (August) and appointed as the 5th Chief of Ordnance of the United States Army.

The decision was based on the large existing stocks of smoothbore muskets in U.S. arsenals, which he argued could be re-rifled in the same manner as the Parrott guns (an assertion which proved incorrect).

Many historians have since decried this decision, arguing the lack of modern arms on the Union side, at a time when the Confederates were buying them in large numbers from France and the United Kingdom, lengthened the conflict by as much as two years.

It is also argued that fouling due to black powder residue would have made it impossible to maintain such high rates of fire under field conditions with the rifles of the time.

Ripley was replaced as head of the Ordnance Department on September 15, 1863, principally because of his continuing opposition to the introduction of breech loading rifles, in disobedience to his orders from President Lincoln.