Taylor once upon a time built a 4 bore which weighed 51 or 52 pounds. It was a smoothbore, however.
We made a mould out of maple & cast up a few balls. IIRC, we used about 300gr. of powder and a tightly
patched ball. We shot it only against the 300 meter berm, so likely 350yards to that bank of dirt. It appeared
to move about a 1/2 yard of dirt with every shot.
Incredibly, the accuracy was decent, at about 3 or 3 1/2 feet, considering no rifling. Upon ignition(underhammer)
the gun would move back about 12". Didn't seem to matter if your shoulder was hard into the butt or not, the
gun simply moved back about 12".
Seems to me, I recall the 'tube' being 2 1/2" across and some 40" long.