There are 2 types of guns in Il-2: machine guns and cannon.
Machine guns fire projectiles from 7.62mm to 15mm. While they are lighter than cannon, unless they hit a vital part of the aircraft, they tend to just leave a small hole in the skin.
Cannon are another matter. They fire explosive projectiles from 20mm to 75mm, and can do serious damage no matter where they hit.
The 7.92mm and 7.7mm/.303 machine guns have very similar performance characteristics.
The Italian and Japanese 12.7mm guns chambered in 12.7x81 and the German MG131 are noticeably inferior to the US M2 and Berezin UB, although the MG131 is acceptable and better than the guns chambered in 12.7x81.
The Japanese 13.2mm has rough parity with the M2.
The largest machine gun of all is the German Mauser MG151/15 15mm.