Cold Steel, I'm not surprised - the M1 was designed to be an astonishingly tough beast to kill, proof pretty much against anything in Soviet service. A rare case of the mil-ind-intel pork acquisition process getting something that's actually really, really good *right*.
Well, mostly, it should have had the 120mm from the get go, but hey
Sadly in game terms if you get that right they become uber-monsters and impossible to deal with without "cheating", e.g. napalm, chemical munitions, thermobarics etc. Which for the original poster, do work wonders!