While I agree with your other points, when inserting anything like inertia or momentum for weaponry for the love of god don't add forced mouse acceleration to this game. Make the max turning speed limited (like the mod/hos tank turrets) or the max acceleration limited. But don't enforce input lag or different order of control on humans as that is not what happens in real life.
In real life you have proprioceptive feedback from your muscles at a speed of about 40ms. Together with a lifetime of training in being able to control not only your default arm but your arm with additional weights added to it as well.
If the input from mouse to output is 1:1 then one can easily learn himself to map the proprioceptive feedback of his real arm with that of the in game character. In case a higher order system is used such as with mouse acceleration then its likely that it takes many years to get your in game arms to work as well as in real life. Its like waking up one morning being unable to use your arms.
If you have to use purely vision information for controlling your character then you'll have to work with the ~200ms input lag that your brain uses to transfer visual imagery to movement. This makes things highly unnatural.
The static MG's are near usable to control already, its made up by the mass amount of bullets they fire and the penetration. But if a rifle would act similar to one of the heavy mg's I would never even touch the game any more. The input control with free aim in iron sights is already unnatural.
I'm up for making things more difficult, but it should not become difficult through the control input. I'd rather see external noise making things more difficult.
Don't be silly Zets, i am not suggesting they add a constant input lag, that would be horrible and make the game neigh unplayable.
What i am suggesting is that a limit be put on how fast you can turn and fire, without denying us the abillity to look around rapidly.
Just like you said, have a max allowed movement speed, and if you move within that speed you have 1:1 mouse movement, allways, BUT, instead of disallowing people to turn any faster than that threshhold (which would be bad, we have a very narrow field of view on a screen and we need the mouse to look around and see what's going on), the weapon simply lags behind the mouse when subjected to rapid movements that go above the speed threshhold (simulating that you can turn your head and eyes faster to look in a direction than you can your entire body).
It would play exactly the same as setting a max allowed turning speed like you suggested, only it would do so without denying us the abillity to look around rapidly, all it is doing for normal weapons is making sure people can't pull off impossibly fast 90 and 180 degree turns (they can look that fast, but the gun won't follow such fast and extreme movement instantly).
Limiting the player to humanly possible turn speeds would go a very long way twords creating what you want, what i also want, more slow and tactical CQB fights, if people can't insta snap their aim at multiple tarets in succession, but have to deal with more human constraints than a 3000+ DPI mouse cranked up to hyper, then running into a building guns'a'blazin becomes harder, more scary, you might get the one guy who's allready in your line of fire, but the guy in the opposite corner, not so much..
Add more player momentum to the mix, so people can't change direction on a dime and do UT style zig-zag circle strafing, and suddenly people are gonna want to play smart instead of playing fast-twitch and reckless.
But for heavy weapons like hipfired MG's, the max allowed turnspeed (from hip, not from bipod) before smoothing sets in needs to be lower than that of a rifle, so the gun does feel heavy and unwieldy, and it does take more time to swing it 90 degrees and fire, or 180 for that matter, else we will continue to have Rambo's running around using thouse guns as an SMG replacement, instead of using them as MG's.