A few years ago (and it must have been quite a few years ago because I can’t find it in my almost ten years of blog posts) the LMI and I were in a surplus store and found a size extra-small flak jacket. You know, one of those Vietnam/Nation Guard vests that look like an olive life preserver. (Technically, I guess thats exactly what it is, come to think of it.) Anyway, being an oddball size it was an easy thing to figure let’s spend $ 15 on it, take it to the range, and see what happens.
If youre the TL;DR type, here it is succinctly: all pistol except super-fast bullets were stopped. No .45 ACP, no 9mm, no .40 S&W, no .38 Special penetrated the vest. The handgun bullets that did penetrate were lightweight (180 gr.) .44 bullets driven at ‘dont try this at home’ velocities and uber-light (95 gr.) JHP’s out of a .357 with similar speeds. Both those bullets were approaching low-end rifle velocities so no surprise there since penetration in conventional firearms is almost always more about velocity than bullet construction. You push a gumball fast enough, it’ll penetrate.
Once we’d exhausted the gamut of pistol calibers and various loadings it was time to move onto the rifles. No surprises there. Shredded kevlar was everywhere. Nothing was stopped.
None of this is going to be a surprise to anyone with a logical mind. Flak jackets were designed to stop irregular shaped pieces of metal at moderate velocities. A .451″ piece of lead traveling at 950 fps is pretty close to that demographic. Higher velocity stuff will just whip through it.
So, in a nutshell, yes they’ll stop most non-magnum pistol rounds and .22. They will not stop anything from a rifle (unless its a rifle that shoots pistol cartridges). Better than nothing, and cheap enough (usually) that almost everyone can at least afford one, but there are better choices if you can swing it.
ETA: It would be criminally negligent of me to forget to mention that if you want real-world info on what will and will not stop a bullet, not armchair theory, then you need to head over to The Box O’ Truth. Shooting bullet proof glass, kevlar vests, sheetrock, car doors….these guys do it all and the information is priceless.
And here’s where they shot a flak jacket and chronicled the results.