Sorry - that's incorrect. Page 40The -6 is for an open-topped tank turret (eg: Puma). A single hatch would be even smaller. One with a fella standing in it ready to bat unfriendly grenades away smaller still. I'd say it's not a valid target unless you climbed up on top of the tank to post it in, while punching the commander repeatedly in the face to keep him busy.
"For a smaller aperture or more difficult target, such as an upper storey window, open topped vehicle or over a high obstacle, subtract 4 from the total.
For a very small appeture, such as an open tank turret or firing slit on a bunker, subtract6 from the total rolled"
As there are specific modifiers I cannot see that Rich did not intend for you to be able to do this? All down to personal preferance as to which rules/actions you allow or not I realise.