Actually, when firing at large targets you add 1 to your roll for every 2" of the target's size rating. For example, if you were firing at a 6" tank, you'd add 3 to your roll. MKII and above guns are really for large targets only. But don't worry, the weapon's *actual* size doesn't really have to correspond to its in-game Size. If you wanted those guns to be antipersonnel weapons, you could still make them MKI.
... and all of this stuff is why I just get my weapon stats from the 2001 rules.