Mutant Future is just begging for a critical hit rule (well, some campaigns are). There are 3 reasons I'm bringing this variant up 1- PC's and NPC's have alot of hit points and this bothers some folks, 2- If a game can survive a little more swingy randomness it's mutant future, 3 - rolling more dice is fun.
In brief when the to hit roll indicates a critical hit could have been scored the attacker is permitted to roll 1d12 versus their HD/level. If this d12 roll is equal to or less then the attackers HD/level they have scored a critical hit. A critical hit does as many additional dice of damage as indicated by the d12 roll.
When could an attack be a critical hit?
The roll to determine if an attack roll is a critical hit is made depends on the die roll of the d20 rolled to score a hit and the type of attack being made.
If attacking with a natural weapon and die roll of 18-20 is rolled and 18 or higher isn't needed to hit the target a critical hit may have been scored, roll the d12 to check.
If attacking with a melee weapon (primitive or advanced) and die roll of 19-20 is rolled and 19 or higher isn't needed to hit the target a critical hit may have been scored, roll the d12 to check.
If attacking with a ranged attack or mutant power and a die roll of 20 is rolled and 20 or higher isn't needed to hit the target a critical hit may have been scored, roll the d12 to check.
How many dice do I add on a critical hit and what type?
If the d12 roll is less then or equal to the level or hitdice of the attacker a number of dice equal to that roll are added to the damage done. If an attacker is level 2 and they roll a 2 on that d12 roll they would do 2 extra dice.
To determine what dice to roll look at the attack you are using. A spear doing 1d6 would do an extra d6 per extra die allowed. A fancy high-tech zap gun that does 7d8 normally will on a successful critical roll 1d8 per extra die allowed. For weapons that do a fixed amount of damage add 20% per "extra die".
Attacks that don't do direct damage dice on a hit roll don't inflict critical hits.
option: In all cases if the total score to hit rolled is over 20 add those extra points to the final damage score. This makes big to hit rolls a little cooler.
I'll be using this rule and the option in my NML:The MutantFront play sessions.
notes to GMs:
Mutant Monsters will be more dangerous then PCs if this critical hit system is used. Don't let on to the players that you are aware of this. If you feel the difference is a bit too much between PCs and Monster types use the save level instead of HitDice for the monster types.
The d12 check keeps things swingy and makes sure higher level characters aren't' always dishing out 50+ points of damage per attack.
High-tech weapons and mutant powers get slightly less oomph from ciritcal hits (comparing average damages rolled) because they have already been tweaked by the wonders of technology to do more damage.