the point-of-aim (centre of scope or scope's cross-hairs) for a specific distance. A rifle and scope should retain its zero as long as possible under all conditions to reduce the need to re-zero during missions. A sandbag can serve as a useful platform for shooting a sniper rifle, although any soft surface such as a rucksack will steady a rifle and contribute to consistency. In particular, bipods help when firing from a prone position, and enable the firing position to be sustained for an extended period of time. Many police and military sniper rifles come equipped with an adjustable bipod. Makeshift bipods known as shooting sticks can be constructed from items such as tree branches or ski poles. Some military snipers use three-legged shooting sticks. Range and accuracy vary depending on the cartridge and specific ammunition types that are used. Typical ranges for common battle field cartridges are as follows: U.S. military Servicemen volunteer for the rigorous sniper training and are accepted on the basis of their aptitude, physical ability, marksmanship, patience and mental stability. Military snipers may be further trained as forward air controllers (FACs) to direct air strikes or forward observers (FOs) to direct artillery or mortar fire. Russian Army From 2011, the Russian armed forces have run newly developed sniper courses in military district training centres. In place of the Soviet practice of mainly squad sharpshooters, which were often designated during initial training (and of whom only few become snipers per se), these new army snipers are trained intensively for three months (for conscripts) or longer (for contract soldiers). The training program includes theory and practice of countersniper engagements, artillery spotting, and coordination of air support. The first instructors are the graduates of the Solnechnogorsk sniper training centre. The method of sniper deployment, according to the Ministry of Defence, is likely to be one three-platoon company at the brigade level, with one of the platoons acting independently and the other two supporting the battalions as needed.<ref>Gavrilov, Yuri "Take a bead: Army gets sniper schools" Rossiiskaya gazeta 19 October 2011.</ref> Targeting, tactics, and techniques Range finding The range to the target is measured or estimated as precisely as conditions permit and correct range estimation becomes absolutely critical at long ranges, because a bullet travels with a curved trajectory and the sniper must compensate for this by aiming higher at longer distances. If the exact distance is not known