So i'm at work and bored out of my mind when this thought popped into my head-Are there any specials rules between the different damage types? I could have sworn i read somewhere energy weapons have a bonus against creatures with the machine trait, but it's very possible my archaic brain just cobbled that together. I'm currently in the process of relearning all of the war hammer 40k rpgs after a two year hiatus so some of the info is mashed together in my head.

The principal difference resides in the critical damage charts; where Energy damage isn't as damaging as explosive or rending damage, for example.

The critical damage tables are the only difference between damage types in the core rules. The Inquisitor's Handbook has rules for altering Medicae checks based on damage type (for example, Explosive wounds are harder to heal than Impact wounds), but I've never bothered using those.

I once created monster, made out of animated stones, and decided to add the effect that Energy and Rending damage inflicted less damage, than Explosive, or Impact.

I believe you are thinking about the 'Silicate' trait from the Koronus Bestiary

Get back to work!

I guess you could always homebrew some damage rules, altough it's best not to get carried away with it and add lots of special rules. We once had to restart a D&D session because of a single fireball atack completly destroyed our wizard. (wand, robes and spellbook gone, disfigured, penalties to just about everything except Wisdom and Inteligence, etc...)

Weapon damage types only impacts two things in game terms; how armour protects the wearer from damage in the case of a primitive or a non primitive weapon, and what crit effects there are on characters that suffer crit damage from a specific type of damage like for example rending.

In addition to weapon damage types, there are also a number of weapon qualities that a weapon can posses like blast(x) where the projectile of the weapon creates a explosion with a diameter of (x) meters. Maybe you where thinking about the quality "shocking"? that becomes less useful againts enemies with more AP, as machines often tend to have in the form of natural armour.