Question about 'tracer fire' used in Apocalyse Now?

In the film Apocalyse Now, a patrol boat carrying the cast and camera crew comes under fire from the river bank. You can see burning ordinance flying through the air, like tracer fire, only it can't be tracer bullets because it moves far too slowly, and in curved paths. Does this represent some actual kind of ammunition? Or was it just a very poor representation of normal tracer fire?