There are old cannons that have been used as bollards: https://westevan.org/bollards/cannonbollards3.htm https://westevan.org/bollards/cannonbollards3.htm > T

There are old cannons that have been used as bollards: https://westevan.org/bollards/cannonbollards3.htm https://westevan.org/bollards/cannonbollards3.htm > The one on the right is a real cannon outside the main gate into the original Chatham Dockyard. It is one of a pair (see the gateway photograph in the gallery below). It had been one of the Royal Navy's biggest smooth-bore muzzle-loading (SBML) guns but when it was no longer fit to be used on a warship it was buried breech-down to protect the brickwork of the gatehouse from damage by carts and other vehicles. The muzzle of this one has been sealed off with a cross-shaped piece of iron. You can also see them as mooring bollards in harbours around the world.