as signals. During the First World War, shrapnel shells and explosive shells inflicted terrible casualties on infantry, accounting for nearly 70% of all war casualties and leading to the adoption of steel combat helmets on both sides. Frequent problems with shells led to many military disasters with dud shells, most notably during the 1916 Battle of the Somme. Shells filled with poison gas were used from 1917 onwards. Propulsion Artillery shells are differentiated by how the shell is loaded and propelled, and the type of breech mechanism. Fixed ammunition Fixed ammunition has three main components: the fuzed projectile, the casing to hold the propellants and primer, and the single propellant charge. Everything is included in a ready-to-use package and in British ordnance terms is called fixed quick firing. Often guns which use fixed ammunition use sliding-block or sliding-wedge breeches and the case provides obturation which seals the breech of the gun and prevents propellant gasses from escaping. Sliding block breeches can be horizontal or vertical. Advantages of fixed ammunition are simplicity, safety, moisture resistance and speed of loading. Disadvantages are eventually a fixed round becomes too long or too heavy to load by a gun crew. Another issue is the inability to vary propellant charges to achieve different velocities and ranges. Lastly, there is the issue of resource usage since a fixed round uses a case, which can be an issue in a prolonged war if there are metal shortages. Separate loading cased charge Separate loading cased charge ammunition has three main components: the fuzed projectile, the casing to hold the propellants and primer, and the bagged propellant charges. The components are usually separated into two or more parts. In British ordnance terms, this type of ammunition is called separate quick firing. Often guns which use separate loading cased charge ammunition use sliding-block or sliding-wedge breeches and during World War I and World War II Germany predominantly used fixed or separate loading cased charges and sliding block breeches even for their largest guns. A variant of separate loading cased charge ammunition is semi-fixed ammunition. With semi-fixed ammunition the round comes as a complete package but the projectile and its case can be separated. The case holds a set number of bagged charges and the gun crew can add or subtract propellant to change range and velocity. The round is then reassembled, loaded, and fired. Advantages include easier handling for larger