Having gotten used to non-variable weapon damage a bit, I'm starting to think about how to handle "critical hits". Typically, I've used a simple "roll a natural twenty and get double damage" model, but that's really gotten a bit stale, and I constantly waffle about whether to add modifiers like strength, magic, etc. to that doubling. On top of that, crits just don't seem to happen very often, and when they do, it seems like the critting player all-too-often rolls a "1" for damage, which immediately turns that critical hit into a deflating, anti-climactic boo-fest.
That's not to say its "all about the damage"; its great when a 20 gets rolled when the player is doing something cool like lobbing a grenade into a giant toad's mouth or participating in an archery contest against the evil baron's head marksman, no damage roll necessary to achieve the desired result. But it would be nice to chop an orc in half once in a while, and sometimes it seems like what you really need are two lucky rolls (first a 20, then a 5 or 6, for instance) to really enjoy that rare critical hit.
So I'm pondering taking the critical hit away from the roll-to-hit mechanic and making it a function of the damage dice instead. I like the old "exploding dice" houserule, so I'm going to try something along those lines.
When a damage result of "6" is rolled, the player gets to roll another d6 and add the result to the previous 6 points of damage. So a "critical hit" essentially means 1d6+6 points of damage, or 9.5 points of damage on average. And 1 in 6 hits is "critical" on average, a grisly figure that lines up better with my gritty pulp sensibilities than 1 in 20.
Whats really interesting about this model is that when you add it to the non-variable weapon damage of two-handed weapons (roll 2d6 and pick the better die), you get an increased chance of a critical hit. Which seems very appropriate for Zweihanders and Dane Axes when you think about it.
And since there is no to-hit roll involved, spellcasters could enjoy the occasional critical hit as well (death by magic missile!, life by cure light wounds!).
So what about those "nat 20's"?
Well, there's a sticking point. I could either just rule a nat 20 always hits (which is kinda special, kinda meh), rule that a nat 20 always hits and is an automatic crit (roll d6+6 for damage), really ramp up the damage potential by allowing a possible third roll for damage (roll d6+6, roll another d6 if that damage result is 6 or greater), or just let the player decide what happens when a crit is rolled ("I impale him to the wall!").
I've looked at a couple of critical hit charts, too. But this is one thing I always enjoy immensely for reading purposes, but absolutely hate in actual play. I want me and the players to decide when a hand is chopped off, not a chart, and I hate stopping the narrative and speed of combat to roll yet again and check yet another chart.
What would you guys do?