Hola,
Short and sweet:
I use Armor as Damage Reduction
I am also using the Torn Asunder critical hit system.
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Problem
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TA's crit system bases "Critical wounds" on how much you beat AC by with your crit (5/10/15 in severity from mild to serious)
In the Armor as DR system, your AC becomes a "Defense Class" which is MUCH LOWER than most ACs, as Armor now becomes DR.
So, basically, when anyone crits right now they easily beat the DC by 10 to 15, making every crit a moderate or serious wound.
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How You Can Help
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I need to figure out how to change the 5/10/15 to a more reasonable number spread for a system where even decently geared players have Defense Classes of only 15 or lower at 6th level.
These two rules are really not compatible without changes to either Defense scaling or attack bonus scaling.
I'm working (slowly) on a system that will include both (though just involving one crit threshold), but without a lot of care on the attack bonus vs. defense score, you're going to end up with the full BAB classes scoring some sort of crit on almost every attack. The advantage I have is that I'm working on something from the ground up, rather than trying to apply changes to an existing system.
Now you could boost the Defense score for everyone... but that's going to make all creatures and players a lot tougher, because they become hard to hit again and have damage reduction as well. Same effect (exacerbated by fewer attacks) if you lower the attack bonus.
Really, it's not a matter of the 10/15 threshold, as it is a matter of the relative scaling. If attack bonuses scale faster than Defense, and crits are determined by exceeding Defense by more than X, crits will become more common as you level, because the d20 value you need to score a crit gets lower and lower.
For converting existing rules, I would suggest using one or the other, but not both at once.
DC is lower but does generally scale similar to AC as it is affected by magical enhancement, any feat that increases dodge, etc, so I think I could make the systems work together by adjusting the threshold.
I just need to do some math to figure out what the percentage chances would be in the normal system and then apply those percentages to the DC system and see if they work out.
What's a good middle ground monster for a 6th level party that I could use for playtesting?