There's a full account of the ballistics of this effect
here.
Here are some diagrams from the same source:
http://www.nennstiel-ruprecht.de/bullfly/fig17.htm
http://www.nennstiel-ruprecht.de/bullfly/fig18.htm
http://www.nennstiel-ruprecht.de/bullfly/fig19.htm
http://www.nennstiel-ruprecht.de/bullfly/fig21.htm
http://www.nennstiel-ruprecht.de/bullfly/fig26.htm
http://www.nennstiel-ruprecht.de/bullfly/fig27.htm
The actual helical motion of the round needn't be large (though it can be for some rounds); a yaw angle of 5-7deg combined with the ejection of the tracer gas from the round at speed will exaggerate the effect. Basically, the tracer gas is ejected at an ever-shifting angle to the bullets velocity vector.
The ingenuity some people will exhibit in ignoring the evidence right in front of their eyes is incredible. The spiral pattern in the trace is obvious and unmistakable in any number of guncam clips.
The zig-zag appearance of US tracers is easily explicable as being due to camera shake. Anyone who has any theories about how camera shake could induce a spiral pattern to the tracers while leaving other straight lines, um..., unspiralled is welcome to share them now.
I would argue that this illusion -- previously unknown to photography -- doesn't exist. It's a real, plain, genuine and obvious pheonomenon known to ballistics since the earliest days of rifled weapons and exaggerated by the tracer smoke.
dduff