so your case is for the common german navy quick firing gun "8,8 cm S.K. L/30" from the 1890s. First german U-Boats where built from around 1906, a long time after development of this gun. There was just an other type of a 8,8 cm, the "8.8 cm L/30 Ubts. Kanone L/30" which used same ammunition as the normal navy gun, but it was only rarely used on Uboats due to its limited power. Most german WW1 Uboats used 8,8 cm L/45 (570 mm long cases) or 10,5 cm Guns. Projectiles for the 8,8 cm L/30 guns are very hard to find.