The general consensus is that it was a 6-inch Dahlgren Rifle, which fired either a 70 or 80 pound projectile, or perhaps both. No actual examples, photographs, or even measurements survive of the 80-pounder Dahlgren rifle. But the Cumberland's was not the only example that experienced a bad day. The USS Hetzel initially had one.
From
Ripley, Warren (1984), Artillery and Ammunition of the Civil War, Charleston, S.C.: The Battery Press, quoted in Wiki:
"
Eighty Pounder Rifle: the first 80-pounder was cast at the West Point foundry with trunnions. Subsequent rifles were cast without trunnions and bronze trunnion band and trunnions were added. The gun was initially well received but soon showed a tendency to burst.
USS Hetzel, a converted Coastal Survey ship armed with one IX-inch Dahlgren and one 80-pounder Dahlgren rifle was engaged in the bombardment of Roanoke Island in support amphibious landings, when the following entry was made in her log for February 7, 1862: "At 5:15, rifled 80-pounder aft, loaded with six pounds powder and solid Dahlgren shot, 80 pounds, burst in the act of firing into four principal pieces. The gun forward of the trunnions fell on deck. One third of the breech passed over the mastheads and fell clear of the ship on the starboard bow. One struck on port quarter. And the fourth piece, weighing about 1,000 pounds, driving through the deck and magazine, bringing up on the keelson, set fire to the ship. Fire promptly extinguished." (
Ripley 1984, p. 106)
It stands to reason that if Buchanan saw that one of the twenty-four Dahlgrens on board the Cumberland had fired a rifled shot, than maybe others on board, if not all of them, would have too.
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Bringing the