Tracer rounds are legal in some states but laws governing this type of ammunition vary, according to The American Marksman. In New York, Internet purchases of tracer rounds must be shipped to a licensed dealer, whereas New Hampshire has no laws governing this product.
Tracer rounds are bullets with a pyrotechnic charge in their base which is ignited by the burning powder in the bullet and creates a bright flame, according to Wikipedia. This enables the shooter to follow the trajectory of the bullet and make aiming corrections accordingly. There are three different tracers: bright tracers, subdued tracers and dim tracers.