I fired 71 shots from an m4 into the offline tigers driver window and never did kill it. yes, i used the 76 mm from about 1000 yards
I don't think they have the driver's windo modeled (i.e. you can't shoot through it and get a driver kill), its just eyecandy.
For the M4A3(76), front shots must be made from about 600yds MAX. Side shots at around 1200yds, and rear shots at around 1500yds.
M4A3(75): NO front shot is possible, NONE. EVERY single shot is a ricochet. Side shots are possible from 400yds or so, and you can turret him in two. Rear shots should be made from no more than D800, and this should be seen as only a way to disable his engine and turret him.
The only one shot kill point I could find was by firing at the top of his turret or hull from some high ground, or by driving right upto him, and pointing your barrel at the engine decking to either side of the turret.