The rapid evolution of the operations from a conventional field war to a trech war gave way to the quick and almost improvised development of trench artillery, whose mission was to send an as big as possible explosive charge to a short range, following an almost vertical ballistic curve. These various and numerous 'trench mortars', sometimes called 'Crapouillots' when French, 'Trench Mortars' when British, and 'Minenwerfers' or 'Ladungswerfers' when German, sent different ammunitions with a small muzzle speed.
These ammunitions being therefore less stressed than the usual shells by the weaker initial acceleration, these 'bombs' could be built with thin steel walls, and give the better part of their volume to considerable weights of powerful explosive charges that could represent up to 50 to 60 % of the total weight !
It is then easy to understand how such weapons were able to flatten tens of metres of trenches by a single hit, and destroy or terrorize their poor inhabitants.
Most of the trench artillery weapons tubes were not rifled, the inflight stabilty being then either random, or controlled by fins, efficient enough at small speeds. In these cases, the bomb was either inserted entirely with its body and fins inside the tube, or a cylindric tail welded to the bomb body was inserted inside the mortar bore (like for the French 58mm 'crapouillots' and their different ammunitions).
In other cases, like the powerful German reglementary minenwerfers, the trench mortars tubes were rifled like a conventional gun, and the ammunitions driving bands were premachined with the matching shapes, and introduced backwards by the gun muzzle.
This kind of weapons evoluated all along the war towards the infantry mortars, and the frontier with the grenade-launcher devices became often unclear. The final evolution brang the weapon to the famous British Stokes mortar, or to pneumatic mortars using compressed gaz for their grenade propulsion instead of explosives.
