use in World War II. The percentage of shell weight taken up by its explosive fill increased steadily throughout the 20th Century. Less than 10% was usual in the first few decades; by World War II, leading designs were around 15%. However, British researchers in that war identified 25% as being the optimal design for anti-personnel purposes, based on the recognition that far smaller fragments than hitherto would give a better effect. This guideline was achieved by the 1960s with the 155 mm L15 shell, developed as part of the German-British FH-70 program. The key requirement for increasing the HE content without increasing shell weight was to reduce the thickness of shell walls, which required improvements in high tensile steel. The most common shell type is high explosive, commonly referred to simply as HE. They have a strong steel case, a bursting charge, and a fuse. The fuse detonates the bursting charge which shatters the case and scatters hot, sharp case pieces (fragments, splinters) at high velocity. Most of the damage to soft targets, such as unprotected personnel, is caused by shell pieces rather than by the blast. The term "shrapnel" is sometimes used to describe the shell pieces, but shrapnel shells functioned very differently and are long obsolete. The speed of fragments is limited by Gurney equations. Depending on the type of fuse used the HE shell can be set to burst on the ground (percussion), in the air above the ground, which is called air burst (time or proximity), or after penetrating a short distance into the ground (percussion with delay, either to transmit more ground shock to covered positions, or to reduce the spread of fragments). Projectiles with enhanced fragmentation are called high-explosive fragmentation (HE-FRAG). RDX and TNT mixtures are the standard chemicals used, notably Composition B and Cyclotol. The introduction of "insensitive munition" requirements, agreements and regulations in the 1990s caused modern western designs to use various types of plastic bonded explosives (PBX) based on RDX. Common Common shells designated in the early (i.e. 1800s) British explosive shells were filled with "low explosives" such as "P mixture" (gunpowder) and usually with a fuze in the nose. Common shells on bursting (non-detonating) tended to break into relatively large fragments which continued along the shell's trajectory rather than laterally. They had some incendiary effect. In the late 19th century "double common shells" were developed, lengthened so as to