Since gamers who have grown up with D&D-style hit points often have trouble grasping a weapon that isn't especially lethal but can kill with one shot, let's imagine a small-caliber pistol with a 1-in-20 chance of killing its target. Does it generally kill its target on the first shot? No, not even close. Would you be scared to take a single shot from it? Yes, definitely. How many shots does it typically take to drop someone? There isn't a good answer for that, because the distribution isn't clustered around a single mean, median, and mode. Half the time the target survives 13 shots before succumbing, but the average number of shots needed to drop a target is 20, and each shot is equally likely to finish the job.
Humans in real life are equivalent to 0-level commoners, which means a weapon that is capable of dealing 8 points of damage on a single blow is a killing shot. I would argue that all humans on earth today are equivalent to a 0-level human. Only in this way should firearms fall into one shot is lethal, as this would also mean a solid blow with a long sword has the same lethal capability. But in D&D PCs are not 0-level commoners, nor are most of their NPC opponents, which makes firearms no more lethal than any other weapon.
My point is a 1-in-20 shot chance of killing it's target is the same as a vorpal weapon. I don't agree that a mundane (non-magical) weapon in the hands of a 1st level character should ever have a 1-in-20 chance of killing any more than a dagger should. Allowing any weapon a one-shot kill capability should cost the same, have the same amount of high magic requirement as a vorpal weapon.
I won't allow my player's 1st level characters to possess vorpal weapons, and would equally not allow them to get a mundane item that is equivalent to a vorpal weapon in killing in one shot.
So if firearms did work the way you suggest, I would house rule it that it could not, because it's an unfair and IMO unrealistic concept.
It's just a different way of modeling damage.
It's the exact same way as vorpal, and a +5 weapon property, by game rules cost 50,000 gp. Most vorpal weapons have a separate + enhancement bonus, which ups the cost to 18000+ gp's more (if it's at least a +3 weapon).
How can any GM justify a 70,000 gp magic item with one-shot kill capability in the hands of anyone less than 10th level?
This is what you are saying.