You mean the Barrett .50 cal anti material/ sniper rifle, .50 mil would be the size of a thumbtack. No it was never used as an anti-tank weapon. The last time a .50cal or even the 14.7mm would have been used as an effective anti-tank weapon would have been the early part of WW2. However by about 1942 these rifles had become totally useless against med and heavy armor.
The Barrett .50 is used for anti material, meaning light vehicles.... such as Jeeps, Trucks, and to "Dispose of" things such as Explosives. It also because of the .50 cal rounds range doubles as a heavy sniper rifle.