Its all about violence rating and mostly for western european and american laws. We won't do anything, that might lead to a change the existing rating.

Ok, understandable, but maybe it would be possible to introduce another way to visually indicate that a gunner is no longer active, for example a destroyed machine gun, shattered cockpit glass or as an easy solution just lowering or elevating the machine gun.

or perhaps just removing the 3d model,as if he fell down the position, or some crew member removed him to try and do CPR inside the belly. COuld lead to a strange effect of disappearing gunners tough

No a single 0.50 cal ;)

Caspar, my original post in this thread was not about wanting more blood :) but more realistic rear-gunners' vulnerability to gunfire

Well... they are not invulnerable. Did you go for steady testing before requesting a change or is it because of impressions/feelings only?
I don't say, all is correct. Maybe there are really wrong values in or at least irregularities. But then its no general problem, rather one regarding single plane types.
The gunners are in many cases somewhat protected by plane parts (to more or less degree), which can reduce the calculated projectile energy or divert them.
Also the gunners can get wounded - not every hit is lethal - and be only reduced in firing abilities/accuracy (since 4.10). They even stop firing, when they are heavily wounded, but are not dead.

It's not that difficult, I get my gunners killed more often than i like; but of course when proper technics are used.... most of the times we (because we try always to fly 4 bombers in diamond) get fighters coming to us from our six.... thats almost a secure kill for our gunners, but if you try the frontal or falling over us from 2-3 or 10-9 you probably gonna hurt us badly or kill our crew members.

Now thats cool! Cooperating bomber jockeys are rare! Where can I find you flying? Online? :-P

Over the weekend I have run a number of tests to check the gunner vulnerability and I think on the whole it is ok. In my opinion most complaints by players about the invulnerability of gunners are caused by the lack of a clear visual indication when a gunner is disabled.

But there is one thing I like to know and I hope Caspar can give me an answer:
Does the game take into account the blast effect of cannon shells? In other words, is a direct cannon hit necessary to kill a gunner or will a near miss also injure or kill him.

Had to forward your question. The answer: Yes, splinters are calculated also for gunners. It is possible to kill a gunner by hitting a very different part of the plane with explosive shells.