9mm rounds seem to be fine for me. In arma, as in real life, armor, backpacks, RPGs, rifles, and anything you can stick between you and a bullet will slow down the rounds. Try shooting them where they aren't wearing body armor (Face, arms, legs. unless they are civilians they're going to wear as much armor as try can carry). The current round penetration system seems good to me :D

What difficulty are you playing on? Try veteran or higher.

It's far to weak, 1 round headshot is not possible with a pistol. Also, the pistol cant even penetrate glass in this game, I don't see how that's even close to realistic.

I'm playing on the highest difficulty. And I feel that if you fired three or four pistol rounds into somebody, wearing combat fatigues and a vest, they'd do more than just... well... flinch.

Also, try using a silencer on a 9mm weapon. Perhaps that's when it's most obvious. At close ranges, a silenced 9mm round would still be lethal if well placed.