Most rounds fired from rifled barrels will follow a tightly-spiralling trajectory.
An imbalance in the round will cause a wobble, much like a spinning top -- the technical word is precession. This wobble will mean the spin axis is not parallel to the direction of travel. Just as spin on a football causes it to swerve in mid-air, the interaction between the wobble, the spin and the airflow over the round will also cause swerve, only in a spiral pattern as the spin axis of the round is itself always shifting.
While modelling all this ballistically is probably not worth the effort, the spiral smoke-trails left by tracer rounds are real and would add a nice touch.
dduff
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