During the Peninsula Campaign, the Army of the Potomac was more than 50% armed with Enfield and Springfield rifles, while the Army of Northern Virginia may have been using close to 40% smoothbore muskets. This may account for the lopsided casualty figures in the Seven Days Battles (15,000 Union casualties versus 20,000 Confederate). By the Battle of Antietam in September 1862, the Army of the Potomac had mostly phased out smoothbore muskets aside from the Irish Brigade, which by choice continued using Model 1842 muskets into 1864. However, ordnance reports show that 40 regiments in the Army of the Potomac still carried .69 caliber muskets at Gettysburg and some as late as the Overland Campaign.