their firearms. The standard calibers used by the world's militaries tend to follow worldwide trends. These trends have significantly changed during the centuries of firearm design and re-design. Muskets were normally chambered for large calibers, such as .50 or .59 (12.7 mm or 15 mm), with the theory that these large bullets caused the most damage. During World War I and II, most rifles were chambered in .30 caliber (7.62 mm), a combination of power and speed. Examples would be the .303 British Lee–Enfield, the American M1903 .30-06, and the German 8mm Mauser K98. An exception was the Italian Modello 91 rifle, which used the 6.5×52mm Mannlicher–Carcano cartridge. Detailed study of infantry combat during and after World War II revealed that most small-arms engagements occurred within 100 meters, meaning that the power and range of the traditional .30-caliber weapons (designed for engagements at 500 meters and beyond) were essentially wasted. The single greatest predictor of an individual soldier's combat effectiveness was the number of rounds he fired. Weapons designers and strategists realized that service rifles firing smaller-caliber projectiles would allow troops to carry far more ammunition for the same weight. The lower recoil and more generous magazine capacities of small-caliber weapons also allow troops a much greater volume of fire, compared to historical battle rifles. Smaller, faster traveling, less stable projectiles have also demonstrated greater terminal ballistics and therein, a greater lethality than traditional .30-caliber rounds. Most modern service rifles fire a projectile of approximately 5.56 mm. Examples of firearms in this range are the American 5.56 mm M16 and the Russian 5.45×39mm AK-74. Types of rifle By mechanism Air gun Spring-piston Break barrel Fixed barrel Underlever Sidelever Overlever Pneumatic (internal pressure reservoir) Pump pneumatic, either single-stroke or multi-stroke Pre-charged pneumatic (PCP) Compressed gas (external pressure reservoir) CO2 High pressure air (HPA) Firearm Single-shot Muzzle-loading rifle, some flintlock and mostly caplock Breech-loading rifle Breechblock rifle, either trapdoor, rolling, dropping, tilting or screwed Break-action rifle Double rifle (list) Repeating Manual Revolving rifle Lever-action rifle, e.g. Winchester rifle, Spencer rifle Pump-action rifle, e.g. Colt Lightning Carbine Bolt-action rifle Turn-pull, e.g. Mauser G98, Lee-Enfield, Mosin-Nagant Straight-pull, e.g. Ross rifle, K31, Mannlicher M1895, Blaser R93/R8 Bolt-release rifle, also known as lever-release rifle, e.g. Verney-Carron SpeedLine Self-loading Semi-automatic rifle Automatic rifle Selective-fire rifle By usage Military and law enforcement Anti-materiel rifle Anti-tank rifle Long rifle Personal defense weapon Precision rifle Designated marksman rifle Sniper rifle