^Effectiveness is defined by its ability to end the threat. A vast majority of SD situations in which a gun is utilized a shot isn't fired. I doubt that a BG is going to check caliber before he decides to advance or not. BGs are typically opportunistic and choose what they believe to be easy targets. When armed resistance is presented, they are less likely to continue viewing the target as easy. There are plenty of examples where actual SD shootings with a .380 have been successful in stopping a threat (up to and including lethal). There are also plenty of examples where larger caliber shootings have not resulted in immediate stops. Plenty of BGs end up found in ERs hours later with multiple shots of 9mm just to walk out a little bit later (hopefully escorted by guys in uniforms and wearing stainless steel jewelry).
A .380 that a person can load, rack, and shoot comfortably and can become proficient with is a much better choice in my opinion than a .40 caliber that the person can't handle. TRUE Effectiveness is more determined by the shooter than it is the weapon.
What he said.