Ugh, I can't help it, I gotta get a bit technical *deep breath*

There are multiple factors that play a role, amount of gunpowder behind a round is one of them.

Others include barrel length, how fast or slow the powder burns, size of the round (mass, caliber, etc.), hardness of the round/penetrator, etc.

Soooo... our POS shooter was using a Keltec Sub2k which has a 16" barrel. Assuming they were using factory loaded 115 gr. 9mm ammo (the most commonly used 9mm ammo in the US), the muzzle velocity would have been around 1,295 fps. However, they're only gaining slightly less than 200 fps over shooting the same round from a full size handgun like the Glock 17 with a ~4.5" barrel has a muzzle velocity around 1,097 fps. Fast sure, but still slow compared to actual rifle rounds which often have muzzle velocities exceeding 2,000 fps, some exceeding 3,000 fps, and at least one exceeding 4,000 fps.

Without diving further down this rabbit hole, the short of it is that body armor rated to stop pistol calibers will still stop this round from a 16" barrel.