Seems to be the myth but the military doesn't think so and neither do I. I've shot thousands of rounds of the stuff; .30 cal, 7.62, .50 cal and 5.56. The tracer element doesn't start burning until the bullet is 25-50 meters out of the barrel. I have fired belts of pure tracer out of M273s, M60s and M240Bs and ran numerous magazines through M14s, BARs, M16s and M4s and never noticed them to be dirtier or more fouled. I still shoot tracers weather permitting; not the most accurate stuff to shoot but fun.
Larry Gibson