AXIS side for me are German/Italian/Japan/Romanian planes.
Im sorry if i didnt mention it.

I flew some test missions P51D vs P51D.
I didnt brake any wing but also i didnt have to.
Few hits and was on fire or thick smoke on engine or with even fewer hits on wing AI couldnt control it.

Unless there's a DM bug for a particular plane, just about all planes are quite vulnerable to being set on fire by .50 caliber bullets.

If anything, they're a bit too flammable, because fires start instantly once enough bullets hit a fuel tank, rather than starting out small and growing as more fuel spills from the tank.

A few planes are quite vulnerable to explosions. In particular, jet aircraft are quite prone to explosion, despite the fact that they either used regular 100/130 octane AvGas (i.e., no more flammable than any other fuel modeled in the game) or less flammable kerosene.

Oddly, the ME-163, which was fueled by a witches' brew of two highly flammable, corrosive chemicals can't be exploded!

The P-38 also loses its engines quite easily.

In general, most aircraft in the game lose their control surfaces far too easily. Yes, the breaking parts effect is supposed to model lethal damage to a particular part, but often it's overdone. It's quite strange to see a giant piece of a bomber fall off due to just a dozen or so .50 caliber bullets.

Is there a flyable plane in the game which has that weapon in a flexible gun mount?

IL2 3M, Pe2 and 3.

Except not the oldest series of the Pe2 in game.

Also engine nacelle guns on Pe-8.

Does anyone know a useable platform for an Italian SAFAT 12.7mm or a German 13mm HMG?

You can add the dorsal gun on IL-4.
SM.79

He-111 H-12

Thanks to everyone for the suggestions for different gun types. I will definitely be checking some of these out.

My new project is to revisit my first experiment using the M2 .50 caliber HMG, but to try firing on target aircraft from angles other than just the front. This should give me a good picture of how all breaking parts work in the game.

Results so far indicate that most aircraft in the game are far more vulnerable to airframe damage than their size and construction would indicate. In particular, just about every aircraft in the game is very vulnerable to damage to wing & control surfaces. Even if you can't break a particular aircraft's wing, it's often incredibly easy to trigger damage textures on it