Some ideas you can use:
While I'm by far not an expert on firearms, I believe that the high end .50 cal sniper weapons, represented by top tier Weapon Law: Firearms tables, might be using a larger bullet and/or more powerful propellant charge than, say, a machinegun of the same caliber. Thus (if that's the case) those tables could be used to represent larger calibers, perhaps up to 20 or 25 milimeters.
The old Outlaw campaign book for Rolemaster (set in the Wild West) contained a Gun attack table which IIRC encompassed such large caliber stuff like Gatling guns and smaller cannons. It was, however, a single table with different max damage thresholds depending on weapon size, and thus much coarser and less detailed than the intricate table progressions of Blaster Law or WL:F.
Consider also "uglying up" the tables by substituting deadlier criticals instead of standard ones to raise the damage done by a weapon, like using Ballistic Shrapnel instead of Ballistic Puncture (though that might be better used to represent special ammo types). You should also make large caliber weapons do secondary or tertiary criticals like Impact, Ballistic Impact or Unbalance.
As for grenade and missile launchers, either use deviation table in Vehicle Manual or resolve the attack on a suitably nasty table and if you cause a critical (i.e. hit) cause a secondary attack on a Grenade table (Equipment manual, I believe), with the damage threshold depending on projectile mass.