Red = edit
The armor pre-damage multiplier allows a weapon to eventually kill anything that does not no-sell the damage type. It may take lots of ammo, but it will die.
I know for sure it damages armor when no HP damage is taken. (Tested unwillingly by plasma against me.)
I do not know if this is correct, but my hypotesis that the Impacting Damage (Before being reduced by armor plate) is multiplied by APD and applied to impacted plate.
That way, if a minigun does 30 averaga dmg, Sectopod has (guessing here) 50% kinetic and a minigun has 5% APD, a shot from minigun will do 0 to 1.5 armor damage per hit. In 10 hits, that would be 0 to 15 damage, 7.5 average. But once it starts damaging HP the armor damage will rise.
A laser minigun, with it's 50 average dmg and Sectopod having 150% Laser, assuming it also has 5% APD, should be even more deadly.
150*5%=7.5 .
Every shot doing 0 to 7.5 armor damage per shot, averaging at 3.75.
Sectopod's Kinetic/Ballistic mod is a flat 100% IIRC. And yes, both of them have a +5% pre-armour bonus. The highest I've seen is the HEAT rounds for the RPG-7, which are a fat +30% - but that's Concussive/Explosive which Sectopods are fairly resistant to.
This page appears to be a semi-complete breakdown of how the damage works.
I'm pretty sure. Because I take into account not only armor, damage multipliers, etc. I do not forget that the Sectopod will not stand with its back turned to you and wait until you put three bursts from 10 meters into it.
That doesn't make the weapon useless, you just need to actually plan around using the weapon. Which is what you should be doing anyway in the circumstances of fighting a sectopod with early-game weapons.
Don't want it to suddenly flip around and gun down your lasergunners? Use other soldiers to eat up its reaction TUs first. Bait it into a favourable ambush situation where your gunners have max TU available for the job themselves. Characters don't get to decide whether or not to take a reaction shot if they have TUs remaining; they either pass the test and react or they don't.
Exploit that.
