The purpose of ammunition is predominantly to project force against a selected target.
Ammunition for infantry refers to the ammunition carried by a typical foot (infantry) soldier. Someone serving in the infantry generally carries, in pouches, bandoliers, etc., one hundred rounds of small-arms ammunition (S.A.A.), and it is usual to supplement this, when an action is imminent, from the regimental reserve (see below). Like any trade, the proper tools are necessary for the task at hand. Infantry need to be provided with the weapons and ammunition to deal with the expected threat, be it another foot soldier, a mounted combatant, armoured vehicle or aircraft.
Every reduction in the caliber (size) of the rifle's ammunition means an increase in the number of rounds carried. One hundred rounds of the Martini-Henry ammunition weighed 10 pounds 10 ounces (4.8 kg); the same weight gives 155 rounds of 0.303 in (7.7 mm) ammunition and at 0.256 in (6.5 mm) the number of rounds is still greater. The regimental reserves were historically carried in six S.A.A. carts and on eight pack animals. The six carts are distributed, one as reserve to the machine gun, three as reserve to the battalion itself, and two as part of the brigade reserve, which consists therefore of eight carts. The brigade reserve communicates directly with the brigade ammunition columns of the artillery (see below). The eight pack animals follow the eight companies of their battalion. These, with two out of the three battalion carts, endeavour to keep close to the firing line, the remaining cart being with the reserve companies. Men also are employed as carriers, and this duty is so onerous that picked men only are detailed. Gallantry displayed in bringing up ammunition is considered indeed to justify special rewards. The amount of S.A.A. in regimental charge is 100 rounds in the possession of each soldier, 2000 to 2200 on each pack animal, and 16,000 to 17,600 in each of four carts, with, in addition, about 4000 rounds with the machine gun and 16,000 more in the fifth cart.
In western (NATO) forces, the 7.62 mm NATO round has been mostly replaced by the lighter 5.56 mm NATO round, which is better suited for automatic fire than the larger round and allows each soldier to carry more ammunition. The larger caliber ammunition is still retained where range and weight of shot is important, e.g. machine guns and sniper rifles.
Other nations, especially forces with former ties to the Soviet Union tend to use rifles related to or developed from the AK-47 with similar sized rounds to the NATO ones. In 7.62x39mm and 5.45x39mm for assault rifles and 7.62x54mmR for sniper rifles and light machine guns.