NFA Modernization Act of 2021 This bill increases from $200 to $300 the excise tax on the transfer of certain firearms. It also establishes a Firearms Trust Fund and transfers to such fund the revenues from the increased excise tax. The bill requires equal 50% expenditures from the fund for gun violence prevention initiatives of the Bureau of Alcohol, Firearms, and Explosives and the Office of Justice Programs of the Department of Justice.