Depends on your tolerance for complication.
I'd say such weapons ignored thresholds. If it was worth putting on a vehical, it would melt armor even if it had to melt it all to penetrate it. (eg. plasma projector 1/10 1 penetration, 10 damage).
You could say that unless armor was specifically Abalative or reflective, there was no threshold. Even low power military grade weapons would melt some armor. Abalatives, being brittle, could have less penetration resistance or armor points/ton, but count as double vs energy.
Reflective coatings would have a high threshold against energy that was ruined if it took any damage.
I would not really want to add too many 'types' of armor as that would overly complicate things.
Speaking of complication, my old system reduced penetration for KE weapons over range. So lasers may not attenuate over the same range an AC-20 might drop off.
|Weapon||Range Size||Short ||Medium ||Long|
|30mm High Velocity||7||10/1 ||8/1||6/1|
|Hawkeye Medium Laser||3||3/1 ||3/1||3/1|
|Lockheed ER PPC||7||3/5||3/4||2/4|
So the PPC has bad penetration, but as a PPC ignores thresholds and will melt its way through armor, delivering 8 points at long range, and 15 at short, basically functioning like standard Battletech with the varied damage at range.
Armor damaging weapons with just enough damage to beat an armor's threshold, combined with high rate of fire become lethal - e.g the 30mm cannon. Its got enough points to damage most armor on each shot, and when fired in high speed bursts can chew armor away quickly. It should be able to chop tanks in half
Still not quite sure how HE should be handled. HE rounds deliver the same damage at most ranges, since they rely on their explosive charge to damage the target. I would think they would be low penetration, high cross section and be pretty much useless against heavy armor.
Shaped Charge warheads would act very similar to energy weapons I believe - they burn through armor with the jet and could be reduced by ablative armor as well.
HESH warheads do not bother to punch through armor, instead attempt to cause criticals/internal damage directly. They would have a special rating perhaps - a 'virtual' penetration value that if it was enough to breach the threshold, would cause a fixed amount of internal damage.
Just thoughts on a subject I've actually though about before