Ballistics is as much an interesting hobby as it is a serious consideration for carry ammo. Been thinking of something lately and decided to crunch the numbers. Sectional density has a direct impact on penetration with bullets of the same type (i.e. Nosler, Sierra, HST, whatever). Going up or down in caliber with the same or close SD *should*, in theory, give similar penetration. Let's say one wanted to mimic the ballistics of the .357 Magnum or 357Sig with the 125gr JHP at 1400-1450fps but in another caliber. I only used .40 as an example here. The sectional density of .357 magnum 125gr SJHP's is 0.140, with the 125gr 357Sig at 0.142. Pretty close, so let's use 0.140 for a nice round number. This bullet moving at 1400fps will yield ~ 543ft/lbs of energy, 1450fps is ~ 583ft/lbs. The closest .40 bullet in sectional density is the 155gr at 0.138, the 150gr coming in at 0.134. You'd have to kick the 155gr out at 1250fps to get ~ 537ft/lbs of energy; the 150gr would need to hit 1300fps for the 150gr to achieve ~ 562ft/lbs. As you can see, the 357Sig can easily reach the velocites to get well over 500ft/lbs of energy (same for the .357 Magnum). However, good luck finding .40 caliber 150/155gr loads getting the necessary velocities to rival the energy of the 357 loads with close sectional densities. The focus of this mental exercise is energy (based on velocity/weight) and penetration (based on SD). Once again I assume the same type/brand of bullet.....in general. For those who may wonder, the 135gr .40 bullet has a lower SD (0.121) than the 115gr 9mm (0.13). So while you can achieve the velocities and corresponding energy out of the 135gr'ers you will come up well short of the penetration of the 125gr 357 loads because of a sub-optimal SD.