Except the Rheinmetall 120mm L/44 cannon doesn't need rifling. Because the projectiles are stabilized by fins, rather than rifling.
And the M1 Abrams had a 105mm rifled bore cannon (made in the UK). We traded up to the 120mm smoothbore on some late model M1s and the M1A1 (through to the new M1A2). And it is not just our choice, either. The Rheinmetall 120mm L/44 is also the choice of weapon for the German Leopard II (although the Leopard IIA6 is upgunned to the new 120mm L/55 cannon), the most common MBT in the Western world, the Israeli Merkava III and IV utilize modified L/44s, the Japa