The 10mm is really easy to get head shots with, I actually used it almost exclusively that on the part you said you used Typhoon on.

Technically revolver is just as useful until you realize there is far less revolver ammo to be found, where as there's a near unlimited supply of 10mm ammo out there.

The other problem with difficulty is AI effectiveness is awkward. I like how their patterns aren't always the same, so it can be hard to predict their actions. However at times an enemy can see you kill someone, and you can jump behind a pillar, and they will lose track of you completely.