In game right now, you can spray at the view ports and kill the driver. Works best with BARs and SMGs, but I have shot the driver out of a hanomag with a single lucky Garand round while the hatches were closed.
If it's not historically correct, perhaps it should go into a bug report? By the way, I brought it up before, but a Dev (or a Betatester) claimed it was intentional for the lowered hatches to not protect the driver 100%.