They stagger all enemies in the area of explosion and usually adds bonus damage versus damage you would do without the perk. Very usefull in cases you would almost 1 shot kill an enemy without the perk. Like fb almost 1 shot kills goblins in vog and with explosive rounds it does it almost every time. Downside is that as said crit kills are harder to achieve, since it counts the crit damage first, and its against most enemies too low to kill so the explosive non critical damage actually kills the enemy. Leading to that all crit kill perks wont trigger, like FBs outlaw and firefly. Id still like to know why explosive rounds add that BONUS damage most of the time. Since it says in the decription it only deals portion of the total damage as explosion, not add anything.