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. The new XM24/L55 gun barrel was 18 inches (45.72cm) longer in comparison to the XM24/L52 barrel used on the M60-series tank. It has a higher chamber pressure, reinforced breach and a higher muzzle velocity. 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 the M68A1 gun was viewed as an economical and practical solution that allowed for commonality in ammunition among the two types of tanks. Secondly was the fact that the M68A1E2 could employ the newly developed M900 APFSDS depleted uranium round that had improved penetration performance in comparison to the M774. These early versions of the M1 Abrams were in active Army service until 1991 and with National Guard units until 1996. M1s built after 1984 were armed with the 120mm M256 and designated the M1A1. Many earlier M1 and IPM1 tanks were refitted with the M256 and their designations changed to the M1A1.