12' seems extreme.
Last weekend in the "Legermuseum" in Delft I saw several large guns called : "Walbus".
They were used from the walls of fortresses or from ships.
Sometimes filled with shred to cause a lot of injuries.
But also used with one large bullet as a sniper gun.
In piece times they could be used for hunting on ducks and other birds, than again filled with shred.
I did not take exact notice of the size, but I guess 8' was approx the longest.
Checked their (museum) website right now, found some pictures, but without size. and size matters in this thread
Best regards,
Willem