Exploding Daggers

Every now and then I come back to combat in D&D and want to fiddle around with it to make it more "realistic".  Of course I realize this is a ridiculous notion on the face of it, but I still like playing around with little combat tweaks.

Today I was thinking about dagger vs. longsword combat.  In traditional D&D (with variable weapon dice) this is a simple d4 vs d8 damage attack.  The fact that a sword's greater reach would make it much easier to land a hit on the dagger wielder is abstracted into the damage potential, instead of being reflected in a bonus to hit.  This has always bugged me, as has the d4 damage for a dagger in general, because it means that you cannot really kill someone with one swing of a dagger.  Maybe you could kill a wizard, or a housecat.  But not an average combatant.  

Part of me says who cares.  D&D combat is very abstract, and an attack roll does not equal a single attack, yada yada.  

And then another part of me came up with this mechanic today.  Wielding a dagger vs. an opponent with a longer weapon gives you a -1 to hit and a -2 penalty to AC.  Dagger damage is now 1d4, explodes on a 4 (reroll the die and add the damage every time a 4 is rolled).

This means that a dagger has a 25% chance of doing 1d4 +4 damage*, a 6.25% percent chance of doing 1d4 +8, a 1.5% chance of doing 1d4 +12 damage...

Looking at a longsword's damage spread of 1-8, it is obvious that 50% of the time the damage will be between 5-8 and 8 is the max.  25% of the time the dagger will do 5-8+ damage and there is theoretically no maximum limit if it kept exploding.  The sword is still definitely at an advantage, but the 6.25% chance of the dagger exploding twice is slightly more probable than a natural 20 on d20 (5% chance) so the dagger has the real potential to wreak memorable havoc.  We all know how often natural 20s roll around, and getting to roll another d4 after already having wracked up 8 damage, with a chance to explode again only 25% away... that sounds like a fun moment during combat.

I would need to playtest it a bit to see how the penalties balance out vs. the chance of extra damage, but at first bush it seems to be pretty simple to use and it scratches my "realistic combat" itch.  If you get inside the reach of someone and you have a dagger, you can brutally murder them.  The penalty to hit is smaller than the penalty to defend, which makes sense to me.  It is hard to defend against a sword swing with a dagger.  It is also hard to jump inside someone's reach and attack with a dagger but I think that is much easier than defending with a dagger vs. a sword.  Granted my experience comes with PVC tubing covered in foam, but we still managed to beat the hell out of each other.

*This is not exactly accurate - these damage examples by % would be correct if the die throw could not keep exploding past each level.