'closing' distance of the target coordinates. NATO has a standard ballistic model for computer calculations and has expanded the scope of this into the NATO Armaments Ballistic Kernel (NABK) within the SG2 Shareable (Fire Control) Software Suite (S4). Logistics Supply of artillery ammunition has always been a major component of military logistics. Up until World War I some armies made artillery responsible for all forward ammunition supply because the load of small arms ammunition was trivial compared to artillery. Different armies use different approaches to ammunition supply, which can vary with the nature of operations. Differences include where the logistic service transfers artillery ammunition to artillery, the amount of ammunition carried in units and extent to which stocks are held at unit or battery level. A key difference is whether supply is 'push' or 'pull'. In the former the 'pipeline' keeps pushing ammunition into formations or units at a defined rate. In the latter units fire as tactically necessary and replenish to maintain or reach their authorised holding (which can vary), so the logistic system has to be able to cope with surge and slack. Classification Artillery types can be categorised in several ways, for example by type or size of weapon or ordnance, by role or by organizational arrangements. Types of ordnance The types of cannon artillery are generally distinguished by the velocity at which they fire projectiles. Types of artillery: Cannon: The oldest type of artillery with direct firing trajectory. Siege artillery: Large-caliber artillery that have limited mobility with indirect firing trajectory, which was used to bombard targets at long distances. See also large-calibre artillery. Field artillery: Mobile weapons used to support armies in the field. Subcategories include: Infantry support guns: Directly support infantry units. Mountain guns: Lightweight guns that can be disassembled and transported through difficult terrain. Field guns: Capable of long-range direct fires. Howitzers: Capable of high-angle fire, they are most often employed for indirect-fire. Gun-howitzers: Capable of high or low-angle fire with a longer barrel. Mortars: Typically muzzle-loaded, short-barreled, high-trajectory weapons designed primarily for an indirect-fire role. Gun-mortars: Typically breech-loaded, capable of high or low-angle fire with a longer barrel. Tank guns: Large-caliber guns mounted on tanks or assault guns to provide mobile direct fire. Anti-tank artillery: Guns, usually mobile, designed primarily for direct fire to destroy armored fighting vehicles with heavy armor. Anti-tank gun: Guns designed for direct fire to destroy tanks and other