While used mostly by the Union army, rifled guns were still a new idea and were not very popular among artillery officers or field commanders. Rifled guns were designated by the diameter of the bore of the barrel in inches. In 1860, the Ordnance Board recommended that half of the existing bronze smoothbore cannons be rifled. But this weakened the cannons to the point they could not stand the strain of firing. So the experiment quickly ended. Ordinarily the barrels of smoothbore cannons only lasted about 500 rounds before having to be replaced. Rifled barrels proved to last much longer in field use. The British-made Armstrong and Whitworth guns were good weapons but there were not enough of them to have any major effect on the war. A problem with the rifled guns was they fired too far for the gunners to accurately see to range their targets. When artillery observers and balloons were used for artillery spotting, this increased the accuracy of the rifled guns. Training artillery crews in the use of rifled guns took longer and was more difficult. General George McClellan was among the Union officers who felt that the American terrain was not well suited for the very long range of these weapons. Jefferson Davis, the Confederate President was of the same opinion. All these factors contributed to the slow acceptance of rifled cannon.