I think rifle caliber rounds would in reality be ineffective for doing any serious damage, much like it is in IL2. In lower calibers there's too much depending on the type of round being fired, because the mass of the projectile is too low to have any real effect by virtue of impact alone.
If all the .303s had was regular rounds then i doubt they would do much damage. If on the other hand they had a mix of incendiary and armor-piercing rounds, it would be like a scaled down version of .50s. A high volume of fire with the correct ammo mix would mean that you stood a good chance of punching holes in some important parts of the target with your AP rounds, then your incendiaries would be able to set things alight.
I don't know what they really used, but i've read a similar discussion about the Hellenic air force intercepting Italian bombers during 1940. Greece had some Pzl.24s, some were also armed with 20mm cannons, but they couldn't do enough damage to consistently bring down an Italian bomber without emptying the entire magazine.
The reason was that most of the ammunition used was regular ball type rounds, incendiaries and APs were in very short supply or totally inexistent.
In one such case a Pzl.24 pilot emptied his guns on the enemy bomber, failed to down him and was so infuriated that he closed within ramming distance. He got the bomber's tail with his prop, they both crash landed and he proceeded to draw his sidearm and take the Italian crew prisoner. Talk about being p*ssed off that your rounds are not up to the job
