The tax is $200 for the transfer of any firearm except a firearm classified as an “any other weapon” which is $5.

An unserviceable firearm may be transferred as a curio or ornament without payment of the transfer tax.

[26 U.S.C. 5811, 5852(e) and 5845(h); 27 CFR 479.11, 479.82 and 479.91]

Source: ATF - https://www.atf.gov/firearms/qa/what-tax-transfer-nfa-firearm