this is because i measure the time between two bursts, not the frames. but there must be some bug, too because the steps a bullet makes should be larger on a slow pc which would mean that the time the bullet needs to fly should be the same on a fast and a slow pc.
update: just checked the code and found out that i had an upper limit for the delta time step of 30 milliseconds. this resulted in the behaviour you described. i raised the limit to 100 ms to compensate. this means that the movement speeds will be ok as long as your framerate is higher than 10fps.