Some history..from Wikipedia..
On 11 December 1974 a
Memorandum of Understanding was signed with the USA for the
possible joint production of a new MBT, after the Americans had
bought and investigated prototype hull number seven in 1973. In
view of the experiences in the Yom Kippur War a much higher level
of protection was demanded than was implemented in the prototypes,
that used heavily sloped spaced armour. The weight class was
increased to sixty tons. Prototype turret number fourteen
was changed to test a new armour configuration, and was turned into
a blockier looking turret as a result of using vertical steel
perforated armour; it already had been much more voluminous than
the turret of a Leopard 1 because of a large internal ammunition
storage locker in the rear.
The Leopard 2 thus initially used perforated armour but not Chobham
armour as often claimed. PT-14 used the 120 mm Rheinmetall gun (as
eventually did the U.S. Abrams). After this two new prototype hulls
and three turrets were ordered, one (PT-20) mounting the original
L7A3 105 mm gun and a Hughes fire control system, a second (PT-19)
with the same fire control system but able to "swap out" the gun
for the 120 mm Rheinmetall design (it was indeed so changed by the
Americans), and one more (PT-21) mounting the Hughes-Krupp Atlas
Elektronik EMES 13 fire control system, with the 120 mm gun.
In the summer of 1976 prototype 19 was assembled and shipped to the
USA, together with hull number twenty and a special target vehicle
to test the armour. The prototype was called Leopard 2AV (Austere
Version) because it had a simplified fire control system. 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 armor
protection. Today we know this was only true as regards a hit by a
hollow charge; against KE-attack the Leopard 2 was almost
twice as well protected as the original M1 (650 mm to 350
mm). The difference with a shaped charge is also
negligible. Its more traditional multifuel diesel engine was also
much more reliable, and provided the same performance. Hull twenty
was fitted with simulation weights, which transpired to equal only
the weight of a turret without armour modules fitted, devalidating
all performance data.
After the comparative test the Leopard 2 hulls were returned to
Germany for further evaluation, but turret 19 remained and was
fitted to the hull of prototype seven, while its gun was changed
for the 120 mm Rheinmetall. In tests until March 1977 it was found
to be far superior to the 105 mm M7 mounted on the Abrams, which
was confirmed by subsequent NATO tank gunnery contests
