Although I don't have any specific statistics, I believe that the 9mm 124 gr. +P loading was intended for pistols, i.e. barrels less than 5 in. In a longer barrel, i.e. subgun, the 124 gr. +P might over expand upon impact and not get the penetration you would want. My understanding from those I talked to, is that the 147 gr. works well in longer barrels like a subgun. It gets the velocity it needs to expand reliably, and penetrates as it was intended to.