dt means delta-time, this is the time passed in one frame (1 iteration of the game logic loop).
Your approach only works for a constant logic frame rate. The interval doesn't remain the same if you change the frame rate. Furthermore, you cannot specify an intutive time as an interval -- with
sf::Milliseconds(200), you know that exactly 5 bullets are shot every second.
By the way, there is the increment operator to increase variables by 1:
++accumulator;