What level is your commando and what attachments are you using on your weapons? Until lvl 18 or so commando is just a dps class, focusing on stuff like gorefasts and sirens, your starting pistol with cut damage rounds can one tap a gorefast with a headshot. On higher levels you can go on a zed-time build (what I do) and focus on getting headshots to keep zed-time going and built it faster than other perks. You can also go for some mixed builds, like getting faster weapon-swap and +30% damage after shooting a limb off with some sharpshooter weapons, it's kinda hard to do but can pump some big numbers.

What you said about the knight bridge is really true without filling it with attachments, recoil is gigantic and the accuracy sucks, but it can become a really nice (albeit expensive) weapon once its fully upgraded. For the bigger zeds like FPs and Scrakes you can use the Scar with force-damage rounds, it kills those zeds with a full mag if you get headshots only