testing between the Royal Ordnance L11A5 and the Rheinmetall Rh-120, the latter was chosen. The turret designs of the two prototypes were modified to allow either gun to be fitted. Although the L11/M256 120mm gun was chosen to be the main weapon of the M1 Abrams in 1979, the improved ammunition for the gun still was 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 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 that the M68A1 could employ the newly developed M900 APFSDS depleted uranium round that had improved penetration performance in comparison to the M774. Prototypes Prototypes were delivered in 1976 by Chrysler and GM armed with the license-built M68E1 version of the 105 mm Royal Ordnance L7. They entered head-to-head testing at Aberdeen Proving Ground, along with a Leopard 2 AV prototype for comparison. The Leopard 2 was found to meet U.S. requirements but was thought to cost more. The testing showed that the GM design was generally superior to Chrysler's, offering better armor protection, and better fire control and turret stabilization systems. These early preproduction prototypes were provisionally armed with the M68E1 105mm main gun while a preferred 120 mm gun and its ammunition were in their design and component development phase. These prototypes used a combination mount that allowed for evaluating both 105 mm and 120 mm guns. During testing, the power packs of both designs proved to have issues. The Chrysler gas turbine engine had extensive heat recovery systems in an attempt to improve its fuel efficiency to something similar to a traditional internal combustion engine. This proved not to be the case: the engine consumed much more fuel than expected, burning . The GM design used a new variable-compression diesel design. By spring 1976, the decision to choose the GM design was largely complete. In addition to offering better overall performance, there were concerns about Chrysler's engine both from a reliability and fuel consumption standpoint. The GM program was also slightly cheaper overall at