to maintain training levels of Guard units as well as using a commonality in ammunition amongst tanks. The M60A1 and A3 variants of the M60 series and earliest pre-production XM1 prototypes of the M1 Abrams tanks are armed with the M68E1 variant of the gun. The M68E1 gun shares the same firing characteristics as the M68. It featured several design improvements including an updated gun hydraulic configuration, a stabilization upgrade for the gun, a gun elevation kill switch for the loader, improved ballistic drive and other component refinements. They were fitted with thermal sleeves on the barrels starting in 1973. During the mid 1970s it was becoming clear that the latest generation of composite based armor was impervious to tungsten carbide penetrators. Work was performed at the Department of Energy's Pacific Northwest National Laboratory to engineer development of depleted uranium as a penetrator material for future ammunition while the Armament Research and Development Command (ARRADCOM) was improving the performance of the 105mm M774 cartridge. The improved M833 round starting in 1983, also using a depleted uranium penetrator to keep the M68E1 gun viable against this improved armor. In 1975 an updated version of the gun, the T254E3 was designed, focusing on the use of chrome plating to improve accuracy. It was used to evaluate improvements to the gun's performance using discarding sabot ammunition. Two guns were built and underwent firing trials at Aberdeen and technical evaluations at the Watervliet Arsenal. Based on the results of these tests the shortcomings of plated bores and gun tubes were found to outweigh any advantage they might offer and the program dropped by May 1976. In January 1978, a program was initiated to develop an enhanced version of the 105mm gun, the M68A1 as a possible alternate weapon for the M1 Abrams. had a higher chamber pressure, reinforced breech allowing the use of high-energy propellants. Although the M256 120mm smoothbore gun was chosen to be the main weapon of the M1 Abrams in 1979, the ammunition for the gun was still not fully developed, thus delaying its fielding until 1984. The early production versions of the M1 Abrams (M1 & IPM1) were armed with the M68A1 for two reasons. First was due to the large number of M60 Patton tanks with the M68E1 gun still in widespread US service in the 1980s and a large on-hand stockpile of 105mm munitions. Fitting the M1 with