After running some Savage Rifts, and looking up a few rules explanations, i'd like to clarify how Mega Damage works in Savage Rifts.
Savage Worlds has a descriptor for damage called "Heavy". This is to represent big vehicles, and large mecha, etc. Using your glock to shoot at a tank has the undesired effect of nothing. (maybe getting ran over?) So the term "heavy" and "mega damage" are interchangeable. In all of the Savage Rifts books it is called Mega Damage, but if you want to bring something in from another SW supplement, then its pretty easy to do so. The general thought was "will this hurt a tank, massive demon, or demi-god", and if the thought was "no", then its not mega damage. All vehicular weapons are MDC.
So this means your Wilk's Laser Pistol does not do Mega Damage. You can use it to hunt rabbits and dear instead of needing a separate old school slug thrower with you for hunting. If you attempt to shoot down a SAMAS Power Armor with your tiny laser pistol, the pilot might chuckle before he starts plugging you with railgun rounds. This also means that your combat cyborg with 8 points of MDC Armor is invulnerable to non-MDC damage. Same if your Ley-Line Walker casts a Mega Armor spell. He's a walking siege tank for a few rounds. ...oh...you thought mages were squishy? BWAHAHAHAHA!!!
Now for the reverse....What does Mega Damage do to non-mega damage?
In Savage Worlds you have a score called "Toughness". This is your damage absorption rating. To damage you an opponent needs to exceed this number. Exceeding it causes you to be "Shaken". Every raise (every 4 points over) gives you a wound. If you are already shaken, then exceeding it normally causes a wound. Mega Damage has no additional effect. It still goes against your regular toughness (which is a combination of your natural stamina and your armor). However, most things that do MDC tend to do allot of it.
For example, our favorite MDC cannon, the BOOM GUN does 4d12+6 damage. The combat cyborg (full conversion) can have a toughness of 22. It's not difficult to exceed that by 16 points (which would be an instant kill). So if you have non-MD armor, expect your toughness rating to be around 6-10. It may still apply to MDC weapons, but you're still probably going to die in a really gruesome manner.