Are muzzleloading cannons classified as destructive devices?

Generally, no. Muzzleloading cannons not capable of firing fixed ammunition and manufactured in or before 1898 and replicas thereof are antiques and not subject to the provisions of either the Gun Control Act (GCA) or the National Firearms Act (NFA).

[26 U.S.C. 5845; 27 CFR 479.11]

Last Reviewed September 17, 2015