Ahah - much clearer once I reread the code
Improved TOW, therefore a dual charge and/or top attack ATGM. These get a bonus of 5+Random(Warhead Size) for the improved technology over straight HEAT added to pen.
There is a 5% chance of any HEAT round getting some of the Warhead Size added as over-pen. There is a 17% chance of under pen due to fuse failures etc.
Cheers
Andy