Heavy guns have no impact on turning speed, but should

With pistol or other small guns it's fairly easy to turn around.
But what the game allows you to do with things like PKM is simpy bad.:
See this for reference https://youtu.be/hAyh6k_gDrE?t=62. Just look how fast is he turning the gun.
Would you be able to turn left/right so fast with such a heavy gun? NO. Heavy guns are:

  • hard to aim higher with
  • hard to turn with
  • harder to run with (which seems to be the only mechanic implemented)

Same goes for RPG, look at this guy turn-right time:
Youtube Video – [00:30..]

what people with high mouse sensitivity can do with heavy guns right now is way off from what it should be.
There should be a penalty for aim/turn speed speed based on the gun weight.

I agree. You've started a great trend on your game to include "weight" of a model by slowing/faster running/ sliding and other goodies. Aiming also should be affected by weapon weight. Weapons have no weight in current state of the game.

Good luck trying to limit the mouse movement speed. Spaceship games do this by introducing a second reticle (one that you aim with, and one that catches up to it by ship rotation and actually shows where your guns are pointing), but i don't see how this could be added to an FPS without making it seem far more artificial than the current method. That's why bigger guns are being balanced in different ways. They are harder to aim as well as to run with already.

It's quite simple. By adding weight of a weapon, when you swing your mouse in given direction, you'd get additional "inertia" move which you'd have to compensate to aim faster with heavy guns.

@Feuerholz please It's called inertia, see for reference:
Youtube Video