Implementing Perfect Crits

Is there any way of setting up perfect critical hits for NPC's? An example like: A short sword critically hits; it would deal 1d6+6 instead of 2d6. I'm mostly using a mix of Beyond20 and D&D Beyond for the player part, but I have no idea how to implement something other than ignoring the 2nd damage roll that the macros and NPC actions automatically do on a crit. Would it be possible to do or would I need something like the API to pull it off?

This might be doable with some trick math. Now, [[ floor(1d20/20) ]] will return a one on a natural 20, or zero otherwise. So, [[  1d6+6*floor(1d20/20)  ]] gets a bonus +6 only when the d20 critted. Thus, [[{[[1d6]],[[6*[[ floor(1d20/20)]]]]}kh1]] works as necessary, outputting either 1d6 or a solid 6. You can test this by replacling the d20 to d4, and making the bonus be 66:  [[{[[1d6]],[[66*[[floor(1d4/4)]]]]}kh1]] now you will only have to spam it a couple of times in the chat box to see that it works as intended. Now make it: CharacterName deals [[1d6+  [[{[[1d6]],[[6*[[ floor(1d20/20)]]]]}kh1]]  ]] damage. Only problem is the d20 roll is buried inside and cant be reused elsewhere, nor even properly displayed. To do that you would have to use the hack of storing the d20 roll in the turn tracker. It is in fact possible to store one variable per token, by exploiting the turn tracker.

Actually, you can extract the d20 value as $[[1]]. But it cant be used in further equations, working as text only. Try it: CharacterName deals [[1d6+[[{[[1d6]],[[6*[[floor([[1d20]]/20)]]]]}kh1]]]] damage, rolling 1d20= $[[1]]

Are you using the default 5E sheet? If so, Oosh had developed a hack for NPC sheets. In the description field for the attack, at the end of it put: }} {{crit=[[X]]}} Where X is the max number(s) for the regular damage dice. So, for your example of the base damage being 1d6, you'd put 6 in place of that X. If it was a greatsword that did 2d6, you'd put either 6+6 or 12 in place of the X. It will give you an unsightly } after the damage roll, but hey, it's easier than rebuilding all the macros manually.