The Maxim guns it has have 600 RPM each while the He-51 has 2 1.2k RPM MGs.
The important stat here is burst-mass per second; that is how damaging the armament is. The russian reserves are better than everybody else, as high as three times as good as british planes, at least two times as good as japanese and german planes. The US planes have two different stats, one is about 70% as effective as the russians, two are even worse than the british. This means that you need three times as many shots with a british plane than with a russian plane to down an opponent.
Russian reserve planes are the most powerful without a doubt, and this is also true for Tier 1. They deal the most damage by a wide margin and turn as well or better than anyone else.