Complex sentence: Over long ranges, the stream of tracer rounds and the stream of ordinary rounds will diverge significantly, due to a tracer bullet's mass decreasing over time, because the tracer material in its base burns and vaporizes.
Split into simpler sentences: Over long ranges, the stream of tracer rounds and the stream of ordinary rounds will diverge significantly. This is caused by a tracer bullet's mass decreasing during flight, because the tracer material in its base burns and vaporizes.