The assist has nothing to do with the Mk 108. Look at the damage model as a 'point accumulation' system and each component has a limit of 100 points. If you're wingman is using .50 cal and he get hits on the wing, fuselage and tail section the points would be allocated like this, for example, wing = 60 damage points, fuselage = 40 points, tail section = 30 points. That's a total of 130 damage points allocated to your wingman for that enemy aircraft.
If you come along and put a 30mm round onto the wing, you're awarded 100 damage points and you see the structural failure of the aircraft. However, your wingman has already registered 130 damage points on the aircraft you disabled so he is awarded the kill.
The damage points accumulate to their maximum and the kill is awarded. If you hit an aircraft that only has 5 points left before it reaches the maximum you could it with a nuclear weapon and still get the assist because the system will only acknowledge the 5 points the aircraft had remaining before failure point was reached.
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On to the second one.... the 30mm cannon, whilst devastating, is not an assured kill from a single round. It is possible for a round to pass through the fuselage and have the aircraft survive... I am pretty sure that I've seen a photo of a Spitfire with a gaping hole in the fuselage from a 30mm round.
Having said that, I have noticed several times lately that I have fighters fly on unabated after I hit them with 30mm and 20mm but this only happens in the Ta 152.
I have film on a 30mm hit on the wing root of a 190 A8 that flew on with no visible sign of damage. I put more rounds into that left wing and it still didn't come off... I had to hose it down with 30mm and 20mm to get it to break up. It did break up into lots of pieces, so clearly lots of hits registered... but it just seems odd that the same hits in a 109 would have killed it immediately. 30mm hits on the wing = instant death when I land those hits in a 109.