It was already in IL2, so should be the same in SOW.

Try taking a late war 109 for a spin and see how fast it runs out of the good stuff (30mm hub cannon). The only planes that seem to have "Hollywood" levels of ammo are the larger American fighters like the Corsair, and P-47. If you opt for the P-47 with extra ammo you can hold down the trigger the entire length of your mission and still land with enough lead in the guns to down a bomber.

I imagine that Oleg and Co. have figured out the exact historical loadout for all the fighters involved and you won't be given an extra round.