The simplest critical hit table (OSR etc.)

I'm running a Dark Fantasy Basic campaign and I'm messing with critical hits a little. This is a simple solution I came up with, but it seems to hit all the goals I'd want for such a system:

- It makes crits meaningful.

- It causes "extreme" results, possibly turning the tide of battle.

- Extreme results are uncommon.

- It takes the the attacker's ability and weapon into account.

- It takes armor into account (and makes armor more meaningful).

- It is very easy to remember even without a table.

- It usually replaces the damage roll, instead of adding another roll.

It has some cool "side effects" too, as you'll notice after you read it:

- It can make 1d6+1 weapons meaningfully distinct from 1d8 weapons, for example.

- You can add a "brutal" or "fast" tags to your weapons that only activates on crits, making a 1d6 axe feel very different than a 1d6 rapier (for example, an axe has +1 damage but a rapier has +1 when attacking again).

- If your character is a tough fighter crits on a 19-20 (for example, using Old School Feats), or even on lower numbers, a natural 20 is still "special" because the margin is higher.

Here is how it goes: a natural 20 causes a critical hit (increased damage) AND allows you another attack (against the same target or other target within reach).

When you crit, roll a d20 and add the margin of success. For example, if you had a +7 to hit and the target AC is 15, your margin is 12 (27 minus 15). Then, compare the results with the following table.

 

9 or less: normal damage.

10+: maximum damage.
20+: max damage x2.
30+: max damage x3.

40+: max damage x4.

Natural 20: roll again with a +10 bonus "ad infinitum".

[Easy to remember: 10 means 100% damage, 20 means 200% etc. If you want to reduce damage a bit, you could keep 10 as maximum damage, and let 20, 30, 40, etc. for double, triple, quadruple damage; notice that double damage is slightly higher on average than maximum damage, especially if you have a positive Strength modifier].

Notice that "normal damage" is an uncommon occurrence, only happening with a clumsy attacker against plate armor or something similar. If you are unarmored, then EVERY crit will deal maximum damage... or more! Nice!

The second part of the crit (...AND allows you another attack) pretty much guarantees that a crit will rarely be wasted.

The combined effects are devastating.

If that's too much for you, a simpler alternative is letting player choose: roll in the crit table OR get a free attack. 

Or even better: deal maximum damage, PLUS choose another attack or roll on the crit table (with a +10 bonus). It allows you to "cleave" through foes if they are weak enough.