Many states have hefty taxes to discourage cigarette use, with proven results. For example, smoking rates in New York (taxed at $4.35/pack) have dropped more than twice the national average since 2003. The ready availability of cheap and stockpiled ammunition makes gun violence easy, as shown in Newton, CT, when Adam Lanza used rounds stored by his mother. This tax will discourage stockpiling and casual use.
There are some 10 billion rounds sold each year in the U.S. Even if this drops 50% and $0.05 is returned to manufacturers to offset implementation, the tax revenue, compounded with stiff fines for noncompliance or smuggling, can be used to create jobs in law-enforcement and mental-health care, which make everyone safer. Most importantly, this does not infringe on the 2nd Amendment.