When firing a gun such as the BK-133 or the BK-18, after each shot the gun requires the player to manually press R in order to chamber the next bullet, despite the fact that this should be done automatically.
The reason I believe this is a bug and not a feature is because it used to be automatic in the past, as well as how on the console editions of the game this works how you’d expect it to work, with the guns all cocking back automatically.