I've actually seen one .50 being fired that produced a visible, corkscrew spiral. The barrel in the gun was shot-out and the tracers looked like that special effect from the Quake rail gun. The spiral started as the round left the barrel and seemed to get wider as it went down range. At all other times it is a straight line of tracers without the "shaking" you see in film.
One time we fired a full belt of tracers for a laser effect (which is a no-no of the type that burns out a barrel easier, or so was the rumor). The only other tracer oddity is that the tracer element sometimes pops out when the round hits and flies off to the side.
The M2 is a mean machine, but rather mundane on the range from a firing excitement standpoint, IMO. The M-60 was always more exciting for me to fire because you got more of a feeling of hosing deadly lead with it up against your shoulder.
Charon