I don't know of any that would fit the bill before 1932. There was a German 13 mm anti-tank rifle introduced in 1918, but it wasn't fitted with a scope and was not suitable for sniping in any case (due to being too heavy and bulky).

If I recall correctly, the US experimented with a .50 anti-tank rifle, but decided that the M2 heavy machine gun (also in .50 BMG) would fill that role better.