Yes a FMJ 17HMR has quite a bit of factors in favor of it defeating thin soft barriers.
Most rounds are designed to expand (penetrate even less) for use against rodents. In fact they are designed to practicly come apart as soon as they hit the rodent, so they would penetrate horribly.
A solid FMJ would have a lot of penetration of a thin soft barrier. A solid copper round even more (and bullets .22 and below are immune to the Federal Restriction on percentage of bullet jacket weight.)
Penetration does not equal penetration. There is many different types of penetration. Momentum penetrates some things better, and velocity penetrates other things better.
A shotgun slug will not penetrate many vests that a 5.7 (just a fast .22) will. Yet shooting something like a deer the same slug would go through one side and out the other, while the .22 would only go a few inches under the skin.
The smallest diameter and hardest round requires the least amount of energy to penetrate something like a vest. The larger the diamater or softer the round the more energy is required to defeat the barrier.
That is why an arrow or dart shape requires the least amount of energy for a level of penetration, and why something like precision designed steel flechettes have huge penetration for very little energy. They are hard steel, and the penetrating point is very small.
Consider that the FN 5.7 round from a pistol is just a .22 at speeds similar to a .22LR high or "hyper" velocity round from a rifle. In fact at 1800 FPS a hyper velocity FMJ .22 would be about the same as a FN 5.7 from a pistol.
Most 5.7 rounds from a pistol go 1500-1800 FPS from a ~5 inch pistol.
A .17HMR round goes 2300-2500 FPS from most HMR platforms, and has a peak velocity around a 18-19 inch barrel.
The .17 diameter requires less energy than the .22 diameter round to defeat a soft barrier just like explained above.
So yes the 17HMR FMJ is a soft body armor defeating round. Since most .17HMR rounds are designed for tiny rodents, the average round poses little threat with rapidly expanding bullets.
However just because a round defeats armor does not mean it will do much to the human body. A .17 or .22 inch hole in thier body (which will have no expansion) will not prevent the individual wearing body armor from drawing and returning a hail of lethal rounds. Such small holes often close up on themselves, further reducing blood loss. Without the rifle wound characteristics such a small diameter wound channel will not likely incapacitate.
So a .17 or .22 would only be effective against body armor from a full auto firearm.
The 5.7 rounds are designed to partialy compensate for what would be horrible terminal performance by bending and flipping in tissue. They only achieve that some of the time with the LEO only rounds.
So defeating armor does not mean the round becomes deadly against those wearing armor. It just means it enters thier body, and does so after losing a good deal of energy defeating the vest. A well trained individual should then be able to return fire with such moderate wounds. They may or may not survive the wounds, but that will usualy be decided long after the gunfight is over and hey have had ample time to return fire.
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