caliber rounds, while range and velocity can easily be varied by increasing or decreasing the number of propellant charges. Disadvantages include more complexity, slower loading, less safety, less moisture resistance, and the metal cases can still be a material resource issue. Separate loading bagged charge Separate loading bagged charge ammunition there are three main components: the fuzed projectile, the bagged charges and the primer. Like separate loading cased charge ammunition, the number of propellant charges can be varied. However, this style of ammunition does not use a cartridge case and it achieves obturation through a screw breech instead of a sliding block. Sometimes when reading about artillery the term separate loading ammunition will be used without clarification of whether a cartridge case is used or not, in which case refer to the type of breech used. Heavy artillery pieces and naval artillery tend to use bagged charges and projectiles because the weight and size of the projectiles and propelling charges can be more than a gun crew can manage. Advantages include easier handling for large rounds, decreased metal usage, while range and velocity can be varied by using more or fewer propellant charges. Disadvantages include more complexity, slower loading, less safety and less moisture resistance. Range-enhancing technologies Extended-range shells are sometimes used. These special shell designs may be rocket-assisted projectiles (RAP) or base bleed (BB) to increase range. The first has a small rocket motor built into its base to provide additional thrust. The second has a pyrotechnic device in its base that bleeds gas to fill the partial vacuum created behind the shell and hence reduce base-drag. These shell designs usually have reduced high-explosive filling to remain within the permitted mass for the projectile, and hence less lethality. Sizes The caliber of a shell is its diameter. Depending on the historical period and national preferences, this may be specified in millimeters, centimeters, or inches. The length of gun barrels for large cartridges and shells (naval) is frequently quoted in terms of the ratio of the barrel length to the bore size, also called caliber. For example, the 16"/50 caliber Mark 7 gun is 50 calibers long, that is, 16"×50=800"=66.7 feet long. Some guns, mainly British, were specified by the weight of their shells (see below). Explosive rounds as small as 12.7 x 82 mm and 13 x 64 mm have been used on aircraft and armored vehicles, but their