This is something that was used IRL in the Syrian Civil War

In the battle for Nablus six months ago, the brave defenders of the city found themselves without artillery of any sort. However, they discovered a cache of hand grenades that had been left behind by the British. Using typical Palestinian ingenuity, they discovered that, with suitably strong wooden or metal beams and elastic cloth, they could rig up a catapult capable of launching the grenade a few city blocks away. These grenade catapults largely took over the role that mortars would play as indirect fire artillery useful in trench or urban warfare. Soon, entire workshops of people in Nablus were busy building 'Nablus Mortars' for use by irregular forces within Palestine.

Length: up to 3m long

Weight: up to 30kg

Materials: wood and metal beams and cloth

Ammunition: hand grenades of various sizes

Range: 50-500m

Cost: not for export at this time

Armour: none