longer able to effectively deal with Soviet medium tank designs, let alone their heavy tanks. These events spurred the United Kingdom to employ a new high-velocity tank gun in 1958, the Royal Ordnance L7 to keep existing Centurion tanks viable against this new Soviet tank design. While the United States began design development of the XM60 tank in 1957 and began user trials of the weapon in 1958. The L7 was specifically designed to fit into the turret mountings of the 20 pounder. This would enable the Centurion tanks to be up-gunned with minimum modifications; hence, the fleet could be upgraded in a shorter time and at a lower cost. The first production tank to integrate the L7 was a single up-armoured Centurion Mark 7 in 1958 which was to prove the future viability of up-armouring and up-gunning the Centurion. From 1959 onwards newly built Centurions incorporated the L7 at production. United States 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 Ground 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. 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 in New York and the gun mounts (M116 for the M60 and M140 for the M60A1/A3) manufactured at the Rock Island Arsenal in Illinois. 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 re-designated as the M48A5. This was done