Hmmm...never heard of a BAR used as a sniper weapon before. Perhaps something that a unit armorer cobbled together?
The M1918A2 BAR has two modes of fire: FAST automatic (550rpm) and SLOW automatic (350rpm). It had no provision for semi auto fire.
The US military did have a high-pressue test .30-06 round (Cartridge, Caliber .30, Ball, High Pressure Test, M1) but I doubt that it was used in the field.
Other types of .30-06 ammo included:
Cartridge, Caliber .30, Tracer, M11
Cartridge, Caliber .30, Armor Piercing, M2
Cartridge, Caliber .30, Ball, M2
Cartridge, Caliber .30, Grenade, M3
Cartridge, Caliber .30, Armor Piercing Incendiary, M14
Cartridge, Caliber .30, Ball, Frangible, M22
Cartridge, Caliber .30, Tracer; M25
Cartridge, Dummy, Caliber .30, M40
Cartridge, Caliber .30, Ball, Match, M72
Cartridge, Caliber .30, Blank, M1909