The only way I could see a pair of blokes with an AT rifle knocking a tank out would be by hitting it right in the rear from less than 50m, getting lucky and severing a fuel line or something. Considering that a Panzer IV stood a good chance of surviving a hit to the front from a 45mm gun at combat ranges, and fair chances if it hit the sides, I doubt a 14.5mm round would fare well against anything much bigger. Granted, you have the lucky shots, and if you shoot something enough times you'll probably disable it, but getting enough shots in without the tank you're engaging, a tank with it, or the infantry supporting the tanks spotting you would need a small miracle. I mean, a 66mm HEAT rocket fired from a LAW would struggle against a T72 unless you were right up it's arse and were incredibly lucky, and a HEAT round is specially designed to defeat armour.