Well, it will penetrate SOME vests. There was a test done recently, I forget by who but it was some agency, cops maybe? They said their vest could not stop a certain round. I can't remember what it was, but I think it was a 9mm load, and it is a common factory round. If you find that article and that load, then get the .357 Sig version of it, well, that would probably do it --to that particular kind of vest. They aren't made all the same.
Russians loaded specials super +P++++... 9mm rounds. Super hot, made to go through vests. Not sure of the bullet composition, sorry, but looks normal. Check Wiki, found it there.
The Sig, it may penetrate SOME lvl 2 vests with FMJ since those vests aren't rated to stop 9mm SMG fire, which is about what a Sig will do, no? With handloads and a long barrel?
Now if you want normal bullets that blow through vests, you may want to look at the 9x25 Dillon. It is like a .357 Sig magnum, uses a 10mm case along with the large primer it takes. Heard of velocities approaching 2000fps. You'll need different bullets though, perhaps those solid copper ones? The others come apart too easy.
I've personally ran a .50 Beowulf through an interceptor vest, put three holes in it and it did some significant damage on the other side. This was with plated Rainier 334gr. bullets, factory loads. Blew my mind. So that means the .500 magnum, it will probably penetrate them too since the ballisitcs are almost identical and so are the bullets.
Same vest, it just flattened 9mm FMJ's, .45's, .40's, but 5.56 M855 zipped on through like a laser (but did nowhere near the damage that Beowulf did!).
I don't need AP abilities for public carry, but I keep an M4 at home and if the SHTF, well, I have my own armor and that M4 and STG58 are sufficient against others. Still, at close range without a plate, that Beowulf has the same effect that a 12ga. would have IF it could penetrate armor. Half inch holes and mushroomed plated bullets nealry an inch in diameter that will certainly go all the way through.
Judging by what that Beowulf does, the 12ga. isn't far off from being able to do it either. Amp up the velocity, reduce projectile weight and diameter making for a bullet with a higher cross sectional density, and BAM! 12ga. AP. You'll probably need a saboted copper slug at about 2000fps --there is stuff on the market for hunters nearly that fast.
Finally, that vest that was tested against the 9mm and failed with that singular round? They don't usually test ALL available ammo against ALL vests. And new stuff is constantly coming out. So they use the NIJ specs, and for lvl3, that is only .44mag and 9mm SMG. So that interceptor vest, it did what it was supposed, and failed at what it wasn't.
. It is unlikely that BG is going to return fire even if he wears bullet-resistant vest.