I played the game as a kid and I've got into the tabletop game lately but I have no idea how the game works mechanically. I don't know what effects my chance to hit, my chance to kill infantry, blow up tanks or what will and won't cause suppression and routs. Looking at the manual, it only seemed to casually mention armour penetration and the like. Anyone know what I'm missing?

Thanks to testing from other members, we have an idea of how penetration and armor works, at least.

So the first issue; the in-game armor values tell you significantly less than you'd think. When an attack hits, the game rolls a 6 sided die; from the result it finds out how much armor the attack met. For an example, if an attack hits a Leman Russ' front armor:
1--55 armor
2--85 armor
3--85 armor
4--65 armor
5--95 armor
6--95 armor
The game lists it's front armor as 80 because it's the average. It's got a funny thing where there are other vehicles with 80 armor but different values. The Lungbursta, for instance; it's strongest value is 90, but it's twice as likely to take a hit as the Leman Russ' 95, but it's next strongest is only 70 (both have 55 for their weakest).

Next, the penetration value is also randomized. Perturabo.235 did some tests that seem to indicate it ranges from about 80% of the listed penetration to value to 1/3+1 above the listed value. If the final penetration value is equal or superior to the armor, the unit dies or takes a wound.

Note that this means infantry armor is practically worthless. I believe Space Marines and War Bosses can something block Gretchin or similarly very low power hits.

Thanks! I feel a bit more confident about taking on the green horde now!