This from the FAQ's ATF website.
http://www.atf.gov/firearms/faq/faq2.htm#m28M28) Are muzzleloading cannons classified as destructive devices? [Back]
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 GCA or the NFA. [26 U. S. C. 5845, 27 CFR 179.11]
PART 179--MACHINE GUNS, DESTRUCTIVE DEVICES, AND CERTAIN OTHER FIREARMS--Table of Contents
Subpart B--Definitions
Sec. 179.11 Meaning of terms.
...Antique firearm. Any firearm not designed or redesigned for using
rim fire or conventional center fire ignition with fixed ammunition and
manufactured in or before 1898 (including any matchlock, flintlock,
percussion cap, or similar type of ignition system or replica thereof,
whether actually manufactured before or after the year 1898) and also
any firearm using fixed ammunition manufactured in or before 1898, for
which ammunition is no longer manufactured in the United States and is
not readily available in the ordinary channels of commercial trade.