The truth is, as with comic books, it depends on the writer.

There's very few hard figures around.

Furthermore, it's worth considering a bullet that has been fired at something tends to have a wee bit more kinetic energy than the average person's punch - yet both are rated as S3 in game.

The best I can come up with is that a marine is to a 20mm cannon what a human is to a 7.62mm rifle.