One shot

while learning how to use firearms you learned how to hit vital parts to kill something in one shot. when you roll a 19 or higher (natural) you kill the creature instantly. however, you can only do that a few times per day before your hands start to shake and you can't seem to aim quite right this means that after 3 shots like this you have disadvantage on all attack rolls. as well, if the creature has above 110 hit points the creature is not insta killed but the normal damage is rolled and multiplied by 10.