Gun Violence Prevention and Safe Communities Act of 201 8 This bill amends the Internal Revenue Code, with respect to the excise tax on the sale of firearms by manufacturers, producers, or importers, to: (1) increase the rate of such tax to 20% on pistols, revolvers, and other firearms and on any lower frame or receiver for a firearm; and (2) impose a 50% tax on shells and cartridges. The bill exempts any department, agency, or instrumentality of the United States from such tax. The bill allocates revenues from the increased excise tax under this bill for law enforcement and public safety grant programs, including programs for research on gun violence and its prevention. The bill: (1) increases the occupational tax on importers, manufacturers, and dealers in firearms and the transfer tax on firearms; and (2) modifies the definition of "firearm" for excise tax purposes to include a semiautomatic pistol chambered for cartridges and configured with receivers commonly associated with rifles and capable of accepting detachable magazines.