when you consider ammo, look at the mass and velocity of the round, but remember that this is greatly affected by the barrel lengh, longer barrel imparts more spin and stabilty to the round, also look to cross sectional area, hollow point vs. non, and even the mass of the gun itself, a basic formula is 1/2mv^2 take 1/2 the mass, and multiply it by the velocity squared, whatever round is greater probably has the greater stopping power, but not always.... you must consider your target, what kind of cloths he will be wearing, do you have to shoot thru objects to get to him, their are so many variables...... basically i agree with the guy who said it is better to hit the target with a .22 than worry about if a mm difference is important, and also .38cal is really just 38/100 of a inch, so convert it over to the metric, i think a inch is 25.4mm so a .38 is roughly 9mm, and a 0.40cal if about 10mm, and a 50cal would be 12.45mm