House rule for critical hits

 Here's a quick house rule for critical hits in my 'rethought' version of D&D (cooked up in response to a comment by Little Odo and this post on Grognardia). 

To recap: 

• shields absorb damage through an opposed D6 roll, unless damage is 6 to a shield roll of 1, in which case the attacker chooses whether to inflict full damage or break the shield; bucklers absorb D4;
• hit points are set at 6 for most first-level characters and monsters;
• All weapons do D6 damage unless the wielder is ogre-sized (D8) or (possibly) very strong;
• there are no damage bonuses (except for magical weapons);
• two-handed weapons allow the wielder's player to pick from the higher of two D6.

I like the balance of 6 HP vs D6 damage. But the problem is that rising HP (even if it's just one point per level) immediately take characters to a point where there's no risk of death from a single orc arrow. I don't want that!

Enter the critical hit. A straight critical on a roll of 20 on a D20 comes up far too often. So here's my rule:

• any unmodified attack roll of 20 allows an 'exploding' damage roll - that is, a roll of 6 on a D6 allows another damage die to be rolled and the total added together. Damage dice may explode infinitely, so long as the highest number on the die is rolled each time. Two-handed weapons explode when either of their 2D6 comes up with a 6. Note that an ogre doing D8 damage needs an 8 to 'explode'; likewise, a giant on D10 or D12 needs a roll of maximum damage. If a dragon hits you on a 20 and then rolls 20 damage, you're in for a short night ...

As a two-handed weapon essentially doubles the chance of exploding damage on a 20, there's a strong incentive to carry one - though at the cost of the D6 absorption that a shield offers. A critical is likely to overwhelm a shield; even if a 6 is rolled against a 6, the extra die will still get through.

Something I really like about this is that it (theoretically) allows even a first-level character to slay a dragon or a giant or similar sack of hit points. It means that the humble orc arrow still poses a risk - however unlikely - to the highest-level fighter. And it frees me up to allow higher-level characters to add hit dice rather than hit points with each level.