It arrived in the US by the end of August 1976, and comparative tests between the Leopard 2 and the XM1 (the prototype name for the M1 Abrams) prototypes were held from 1 September at Aberdeen Proving Ground, lasting until December 1976. The US Army reported that the Leopard 2 and the XM1 were comparable in firepower and mobility, but the XM1 was superior in armour protection. (Today we know this was true as regards a hit by a hollow charge; but against KE-attack the Leopard 2 was almost twice as well protected as the original M1.)