A mortar is a weapon that fires explosives shells, the one you see here is an emptied shell or bomb.  They are fired at close range as they do not have a long range.  Mortars have been used since the Medieval times in various shapes and sizes.

This mortar would have been deployed with a German “Granatenwerfer 16” that could throw the grenade up to 460 meters.  This grenade launcher was a spigot-type mortar that consisted of a solid rod instead of the hollow mortar tube.

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