If there was enemy infantry, the hatches would be closed. They lock from the inside. There is only one case I have ever heard of of firing inside the hatch, and that was after the german soldier banged on the hatch in a similar way he had seen russian infantry doing. The guy opened it, was shot, and then the german tossed in a nade.
Other then this case, it is unheard of. You can't just walk up to the tank, climb aboard, and pry open a hatch.
As for firing through slits, the slits are smaller then a bullet for the most part, as well as being equipped with bullet proof glass. They were not just open slits.