I had developed this ruling and have been implementing it for awhile now, and the players have seemingly loved it thus-far. The Exploding Crits ruling basically works like any exploding dice system: if you roll the max amount, keep rolling until the number is no longer achieved, adding in all previous results to a pool. In the case of the d20 system, if a critical (natural 20) is obtained via rolling, keep rolling until you don't obtain a natural 20 result.

For example, Impernius goes to strike an opposing goblin with his shortsword, and lands with a natural 20. Impernius would then proceed to roll the die again, having already confirmed the critical hit. If another natural 20 is rolled after the first, the die is rolled again; for the sake of this demonstration, only two natural 20's will be rolled. The resulting two natural 20's (criticals) in a during this one attack results in dice damage being tripled, or doubled per standard with another effect such as lopping the opponent's arm off ensuing.

Let me know what you guys and gals think!

Someone here the other day posted the idea that critting, in their campaign, maxes out the damage dice. Thus, critting suddenly becomes very explosive. Rogues turn into fully realized assassins, and fighters really stumble their foes. I wasn't sure if he utilized the rule on spells, though.

I personally just double the dice's base damage on a crit; explosive, world-shattering crits are neat, but a lot of my players are still getting used to the game and I'd love for them to go into other campaigns without much need for rule explanation.

I always run same with my group if they roll a 20 no need to roll the dice they just do max damage.

Quote from Naith_Ungol >>

I had developed this ruling and have been implementing it for awhile now, and the players have seemingly loved it thus-far. The Exploding Crits ruling basically works like any exploding dice system: if you roll the max amount, keep rolling until the number is no longer achieved, adding in all previous results to a pool. In the case of the d20 system, if a critical (natural 20) is obtained via rolling, keep rolling until you don't obtain a natural 20 result.

For example, Impernius goes to strike an opposing goblin with his shortsword, and lands with a natural 20. Impernius would then proceed to roll the die again, having already confirmed the critical hit. If another natural 20 is rolled after the first, the die is rolled again; for the sake of this demonstration, only two natural 20's will be rolled. The resulting two natural 20's (criticals) in a during this one attack results in dice damage being tripled, or doubled per standard with another effect such as lopping the opponent's arm off ensuing.

Let me know what you guys and gals think!

I'd love it as a player until the moment a monster did it to me, then I'd hate it. This might seem like a fun addition for the players right now, but it actually works out to their detriment in the end. I personally wouldn't do the max damage thing mentioned in rtlsrd's post either, for the same reason.

• Consider that statistically, a GM is more likely to roll crits than the players are once you've accounted for the amount of monsters they are rolling for and the amount of monsters that have multiattack along with ways to get advantage (eg. pack tactics).
• What about monsters with extra poison damage, or sneak attack or the equivalent of a paladin's smite (or any number of additional damage riders)? That gets doubled too. 
• Also consider that it's a much smaller deal if a PC drops a monster in one hit than it is if a monster drops the PC in one hit. Losing one monster is no loss to you (you're GMing to create fun for your friends, and BOY was that moment just now fun. That's a win). Being knocked out of the combat, and potentially dying, is a big loss to the player. At best they lose a round or two before being cured. At worst their character dies. The likely middle ground is they have to sit there bored with nothing to do until someone can heal them at the end of the battle. 
• Finally, consider the increased chances for instant death in an exploding damage system, however statistically unlikely it may seem. No one is going to walk away from that feeling satisfied that their character was killed in an instant.  Worse still, this is much more likely to happen at low levels when the character is still fresh and new, and the player less likely to be ready for a change.

5e Criticals are weaksauce, make no mistake. Being able to roll a critical that does less damage than the attack you did last turn is pretty crappy. My personal feeling on this is it's far too much in the other direction, though. In my own games, I simply double the dice and the ability bonus. That way at least in the absolute worst case, the critical does solid damage compared to all but the luckiest regular attacks. 

The dice multiplied do not fall under exploding rules, only the d20 roll. You do make valid points, and I appreciate your input.

My apologies. I also forgot to mention that the players have access to this system, and not so much the monsters (if at all.)

When I DM crits do max base damage, then you roll all bonus dice. That way you always do more damage with a crit that you could do with out. I hate rolling a 20, and suck at the damage roll. 

My only problem with maxing dice damage is it cuts out randomization, but I have been on the end where I rolled less for critical damage in comparison to standard rolling.

What if you had "advantage" on the crit rolls eg. You crit with a rapier, you roll 4d8 pick the highest two then add modifiers 

Eh, it's an interesting idea, but personally I don't like game systems like World of Darkness that rely on "exploding" dice, as it only slows the game down by making the numbers of dice rolled inconsistent; I much to prefer to know "okay I roll one d20 to hit, then XdY for damage" and that's it.

In theory I'm a fan of maximising damage like rtlsrd and AdamAldred mention, however I would say that the correct way to do it is to take the number of base dice, multiply by the size, then add the number of dice; this gives you the same average damage as rolling double the dice. For example, 4d6 averages to 14, so a critical on 8d6 averages to 28, 4 times 6 plus 4 also gives you 28, skipping the need to roll anything. The problem is that most people (myself included) aren't great at arithmetic, especially a few drinks of the fermented variety have been imbibed; one way around this is to work out your crits in advance, i.e- work them out for your most common attacks and spells, and keep a note of any others that come up during a game, so you can just look up the correct value and announce it.

That said, it's not as fun as actually rolling dice, so really the best thing to do is just make sure you have enough d8's, d6's etc. to roll your most common attacks/spells/whatever, then if you get a critical hit all you need to do is roll the correct number twice and do some addition. Plus if you're going to go the calculated damage route, you might as well just do what monsters do and use average damage for all your attacks, and only roll so if it's a critical or not, though some DM's don't bother with criticals for monsters (i.e- a critical is just a hit regardless of AC, no double damage).

Of course since the pandemic started I haven't played any in person sessions (even with lockdowns lifted it's been too difficult to organise any as people are never in the right places at the right time, though we might manage something at Christmas this year I hope), so we've mostly just been using video chat and Avrae, which handles rolling of criticals (and adding up totals) for us. If we do manage an in-person session it's probably going to be so slow to resolve everything.