Secretary of War Simon Cameron authorized the formation of an Irish Brigade in September 1861. Originally comprised of New Yorkers, its ranks were joined by men from Massachusetts and replenished with Pennsylvanians after the bloody losses at Antietam. These losses were incurred partly because of the weapon that the men carried, the Model 1842 smoothbore musket – an obsolete weapon that was largely phased out by the Union Army. Irish Brigade Brigadier General, Thomas Francis Meagher, wanted his men to fire buck-and-ball shot (a .69 caliber musket ball with four smaller balls), which produced a deadly shotgun-like blast, most effective at close-range. This charge could not be used with the rifles that were issued to the two Irish Brigade Light companies.