Green’s Dictionary of Slang

(US) an M2 machine gun or Browning .50 caliber heavy machine gun .

1982		S. Dunstan Vietnam Tracks: Armor in Battle 1945-75 96: [An] M2HB Browning atop the commander’s cupola, and a second Ma Deuce’ forward of the turret rack.
1994		H. Halberstadt US Navy SEALs 142: The PBL mounts a ‘Ma Deuce’ M2.50cal .
2007		Field & Stream Feb. 70: MEET MA DEUCE In 1919, John Moses Browning developed a heavy machine gun [...] designated the M2, it is very likely the most successful military weapon ever used by any armed force.
2021		T. Pluck Boy from County Hell 26: He’d built his own rifle from a Ma Deuce, and the .50-caliber rounds cut through the jungle like heart-seeking missiles.