Ouvrage du Hackenberg: block 3 was armed with a retractable turret with two 81 mm mortars. It was impossible to hit it with flat trajectory fire which can't be said for the GFM turret of the block. These turrets proved to be very vulnerable for AT gun fire. However these guns could enemy take out a turret when firing at close ranges. With the artillery of the works this was not possible. Especially the 81 mm mortars were very effective against the German AT guns that tried to get to close.