If you do a Google image search for “hydrostatic shock” you’ll pick up a bunch of pictures of bullets going through ballistics gel and causing a big cavity as they do so. That big cavity is caused by the passage of the bullet and the not compressible nature of liquids, but after the bullet passes it generally slurps right back down to the “permanent wound channel.”
So what is “hydrostatic shock” really? Well, in WWII a surgeon noticed that wounded servicemembers were going into “shock” from bullet wounds, and opined that maybe this was from “hydraulic effects.” Later, studies on pig carcasses determined that the hydraulic effect distant to the wound channel was negligible. Later still, other scientists hooked up live pigs to EEGs to monitor brain activity, then shot the pigs and recorded the difference in brain waves and activity location. They discovered, that animals shot, even in an extremity, do go into “shock.” Other scientists still believe that this change in brain activity is due to pressure waves from the bullet impact site, but I’m skeptical and think that if you slam the impact site with a sledgehammer you’d see a similar change in brain activity since the scientists with the EEG didn’t differentiate a pain/injury response separate than that of a gunshot wound.
So, from a physical, biological perspective, “hydrostatic shock” isn’t a real thing. You have the permanent wound cavity that crushed and destroys tissue, and you have the temporary wound cavity that doesn’t destroy tissue but moves it around a bit. Both of those things you can measure. Ironically, if the temporary wound cavity is larger than the animal, such as a prairie dog hit with a 55gr FMJ from a 22-250, you’ll see the temporary wound cavity turn into a “pop” of red mist as the animal turns into chunks.
Now, blast waves on tissue is a real thing, and can produce negative medical outcomes up to and including death. Being too close to an explosion is a significant emotional event (speaking from experience here). But getting punched in the chest or head at the wrong time or angle can also kill you, and the energy levels there are MUCH lower than those produced by most service pistols. Still, most people shot with pistols don’t die, but many are incapacitated by “shock” if they aren’t also in a fight where they don’t notice being shot at all because of a surge of adrenaline from the “fight or flight” response.
So…hydrostatic shock isn’t a real thing. Don’t waste your money on anything that advertises superior hydrostatic shock effects. There are no pictures of “hydrostatic shock” on the internet, only pictures of a “temporary wound channel” in ballistics gelatin. The fact that animals can go into “shock” when injured (from a fall, an attack by another animal, a gunshot wound, or even heat exhaustion) is perfectly normal, and has everything to do with the standard biological response to injury rather than some mythical effect of a bullet.