26 USC 5811 - Transfer tax

(a) Rate 
There shall be levied, collected, and paid on firearms transferred a tax at the rate of $200 for each firearm transferred, except, the transfer tax on any firearm classified as any other weapon under section 5845 (e) shall be at the rate of $5 for each such firearm transferred.
(b) By whom paid 
The tax imposed by subsection (a) of this section shall be paid by the transferor.
(c) Payment 
The tax imposed by subsection (a) of this section shall be payable by the appropriate stamps prescribed for payment by the Secretary.