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.
it would be nice if one of you mac users could do a quick test for me.