There was a time in our history when a citizen
could own a cannon. Now if he shot it at his neighbor
or the blast from it broke his neighbor’s window then
he was subject to civil recourse in the judicial system.
From his neighbor or if he killed his neighbor then
the local authorities, his peers.
Yes, owning a black powder cannon is still unregulated. I think you are referring to Letters of Marque that gave Congress the power to allow privately owned ships with cannon to act as a naval militia.
But owning a cannon that shoots modern explosive rounds is legal but highly regulated. The cannon must be registered as a destructive device and every round must also be registered. Woe be it to the person with 20 rounds registered and 25 rounds in his possession.