The main gun for the M60 tank series was chosen after a comparative firing test of six different guns carried out on the Aberdeen Proving Grounds in 1958. The factors evaluated were accuracy, the lethality of a hit, rate of fire, and penetration performance. Based on these tests, the 105 mm T254E1 was selected, modified to the T254E2 and standardized as the Cannon, 105-MM Gun, M68. They are licensed built US variants of the L7. The T254E2/M68 used a vertical sliding breechblock instead of the T254E1's horizontal breechblock. Until American-made barrels could be obtained with comparable accuracy, British X15/L52 barrels mounting a concentric bore evacuator on the barrel were to be used. US built XM24/L52 barrels (length 218.5 inches) fitted with an eccentric bore evacuator were used for the M60-series starting in June 1959 but retained interchangeability with the British X15/L52 barrel. All of the US guns and XM24 barrels were produced at the Watervliet Arsenal, NY and the gun mounts (M116 for the M60 and M140 for the M60A1/A3) manufactured at the Rock Island Arsenal, IL. US M68 guns were fitted with an eccentric bore evacuator instead of a concentric model in order to provide more clearance over the rear deck of the tank. The original variant of the M60 tank was equipped with the M68 gun using the M116 mount. Additionally, many M48A3s armed with a 90mm gun that were in NG-CONUS service with the Army National Guard were retrofitted with the M68 gun and redesignated as the M48A5. This was done to maintain training levels of Guard units as well as using a commonality in ammunition amongst tanks.