1. Velocity needs to be considered in the context of bullet weight. It's easy to get a lot of velocity with a very light bullet, but that's not necessarily beneficial as very light bullets can be poor penetrators.
2. Penetration needs to be taken in the context of expansion and also in the context of what is desirable for self-defense. Excessive penetration (especially without expansion) is considered to be undesirable by many--the FBI protocol grades a loading down if it penetrates beyond 18".
3. In general, trying to compare two loadings with only a single number is pointless unless all the other parameters (velocity, bullet weight, bullet construction, expansion, caliber, penetration) are identical, or nearly identical.
4. Because of the wide variety of loadings available, trying to draw conclusions about an entire caliber by looking at just a few loadings (or even just the loadings from one company) can be somewhat misleading, especially if the above 3 suggestions are not being followed.
9mm beats 40 as reported by Vista in steel, wallboard, and plywood.
1. That resource allows one to compare up to nine 9mm loadings with up to four S&W loadings. Every single loading in each of those calibers provides different results in terms of penetration and expansion.
2. If you are in LE, then penetration through steel, plywood and glass are more likely to be applicable considerations. For civilian self-defense and even for typical LE, the bare gelatin and clothed gelatin figures (maybe wallboard) are likely to be more applicable.
If your goal is to prove that you can find a 9mm loading that penetrates more than a .40S&W loading, you can certainly do that. If you only care about penetration, then compare any FMJ 9mm with an expanding .40S&W and you're done. 9mm FMJ tends to penetrate about 2 feet in bare gel, expanding .40S&W probably won't even make 18" in bare gel. Of course, that's because most expanding ammo intended for self-defense is designed to penetrate less than 18", but it does simplify the search if you just want to compare penetration.
Similarly, if you want to prove that there's a 9mm loading that goes faster than a .40S&W loading, just look for a light bullet 9mm loading--you can probably find something in 90gr that will go close to 1500fps. Then compare it to a heavy bullet load in the .40S&W--maybe 200 grains--I would guess that you could, without too much trouble, find a load that didn't even make 900fps in that caliber with that bullet weight.
The problem is that kind of comparison doesn't really tell you much about how the two calibers compare because it is looking at extremes in both calibers and is only looking at one parameter at a time instead of looking at what kind of tradeoffs that result. For example, a very light bullet will likely result in low penetration numbers. A very fast bullet for caliber could be more likely to result in bullet failures.
Getting an overall feel for the differences in two calibers is not simple. Fortunately it's not really necessary--you can just pick a loading that gives you the performance you want in a caliber you like (within reason--if you go below 9mm, you may not be able to get both adequate expansion and penetration) and you're good to go.
